feat(08.1-01): rebuild all fixtures and bot graph builders through the ARG3 path

- mini-graph-arg2.bin: the frozen ARG2-era bytes preserved once as the
  D-12 decode-rejection vector (never regenerated — the arg1 precedent)
- build-mini-graph.mjs: landmark stage out, CH stage in — contractGraph
  over the 25-node grid (41 shortcuts, 1.64/node, 0 budget hits) with
  fail-loud invariants: total-order rank, grid shortcut band, upward
  shortcuts, overlay-exact constituent weights (f64 replay, f32 compare),
  strong connectivity; deterministic ARG3 bytes
- graph.test.mjs: committed-fixture G4 added (ARG3 magic/version, 25
  nodes, 80 edges, rank 25, >=1 shortcut, ARG2 vector -> null); G7
  restored (full ARG3 pin)
- astar.test.mjs: A1-A6 restored — the 625-pair Dijkstra equivalence,
  oneway law, deriveSteps, depart/arrive, classifyTurn all green against
  the CH query on the committed ARG3 fixture
- snap.test.mjs: S1-S6 restored on the ARG3 fixture; S3's synthetic
  graph rebuilt as the ARG3 ch-block star
- webhook.test.mjs: landmarkBlock -> chBlock (contractGraph); corridor
  gains the fast nonstop chord (a pure line starves to zero shortcuts);
  sparse star keeps node ids 0-hub; RT9's split carrier is now two
  disconnected decodable stars (the honest A\* miss)
- guidance-cron.test.mjs: GC6b's 2-node graph becomes the lowest-id-mid
  3-node star (a 2-node graph cannot carry a valid shortcut)
- routing 87 passed / bot 126 passed; routing+bot+web typechecks green

routing/test/fixtures/build-mini-graph.mjs

@@ -3,24 +3,28 @@
  * build-mini-graph.mjs — builds the committed mini-graph.bin fixture.
  * ----------------------------------------------------------------------
  * A MAINTAINER-TIME REPO TOOL: regenerates routing/test/fixtures/
- * mini-graph.bin — the A* correctness ground truth (plan 06-01) and the
- * snap/astar suites' shared fixture. NOT a runtime dependency; nothing in
- * bot/src/, web/src/, or routing/src/ imports it. Tests consume the
- * COMMITTED binary; only a deliberate fixture redesign re-runs this.
+ * mini-graph.bin — the routing correctness ground truth (plan 06-01) and
+ * the snap/search suites' shared fixture. NOT a runtime dependency;
+ * nothing in bot/src/, web/src/, or routing/src/ imports it. Tests
+ * consume the COMMITTED binary; only a deliberate fixture redesign re-runs
+ * this.
  *
  * PROVENANCE (Phase 8 plan 08-01): routing/test/fixtures/mini-graph-arg1.bin
  * holds the frozen ARG1-era bytes preserved expressly as the v1
  * decode-rejection vector consumed by the G-suite — it is NOT regenerated
  * by this script, ever.
+ * PROVENANCE (Phase 08.1 plan 01): routing/test/fixtures/mini-graph-arg2.bin
+ * holds the frozen ARG2-era bytes preserved expressly as the ARG2
+ * decode-rejection vector consumed by the G-suite (D-12) — it is NOT
+ * regenerated by this script, ever.
  *
  * Usage:   node routing/test/fixtures/build-mini-graph.mjs
  * Exit:    0 on write; throws on any invariant violation (never writes).
  *
- * ARG2 (plan 08-02): the fixture carries a tiny real landmark stage —
- * L=2 anchors via farthestPointAnchors + quantized c-metric tables via
- * landmarkTables (the SAME pure module the real build script uses), passed
- * through the ONE encodeGraph authority. Deterministic: identical output
- * bytes on every re-run.
+ * ARG3 (Phase 08.1, D-12): the fixture carries a tiny real CH stage —
+ * contraction via contractGraph (the SAME pure module the real build
+ * script uses), passed through the ONE encodeGraph authority as the `ch`
+ * block. Deterministic: identical output bytes on every re-run.
  *
  * Design (pinned by 06-01): a hand-built 5x5 grid in a ~0.002 deg x 0.002 deg
  * bbox near KL center (lat 3.1500–3.1520, lng 101.6900–101.6920):
@@ -34,7 +38,7 @@
 import { writeFileSync } from "node:fs";
 import { encodeGraph } from "../../src/graph.ts";
 import { haversineM } from "../../src/geo.ts";
-import { LANDMARK_QUANTUM_S, farthestPointAnchors, landmarkTables, landmarkCoverage } from "../../src/landmark.ts";
+import { contractGraph, CH_SHORTCUT_EXPLOSION_RATIO } from "../../src/contraction.ts";
 
 const GRID = 5;
 const LAT0 = 3.15;
@@ -92,27 +96,86 @@ reverse.flags = 1;
 
 if (edges.length !== 80) throw new Error(`expected 80 directed edges, built ${edges.length}`);
 
-// LANDMARK STAGE (plan 08-02): the fixture regenerates as ARG2 THROUGH the
-// one encode authority — the same farthestPointAnchors + landmarkTables the
-// real build script (08-04) will run, at a tiny L. Fail-loud invariants
-// before any write (the `edges.length !== 80` style).
-const LANDMARK_COUNT = 2; // tiny L: the 5x5 grid needs exactly 2 well-spread anchors
-const landmarkInput = { nodeCount: GRID * GRID, edges, quantumS: LANDMARK_QUANTUM_S };
-const anchors = farthestPointAnchors(landmarkInput, LANDMARK_COUNT);
-const { to, from } = landmarkTables(landmarkInput, anchors);
-if (anchors.length !== LANDMARK_COUNT) throw new Error(`expected ${LANDMARK_COUNT} anchors, got ${anchors.length}`);
-if (to.length !== LANDMARK_COUNT * GRID * GRID || from.length !== LANDMARK_COUNT * GRID * GRID) {
-  throw new Error("landmark tables are not L*N row-major");
+// CH STAGE (Phase 08.1): the fixture regenerates as ARG3 THROUGH the one
+// encode authority — the same contractGraph the real build script (plan 02)
+// runs. Fail-loud invariants before any write (the `edges.length !== 80`
+// style), per RESEARCH §Pattern 4.
+const ch = contractGraph({ nodeCount: GRID * GRID, edges });
+const rankValues = [...ch.rank];
+if (new Set(rankValues).size !== GRID * GRID) {
+  throw new Error(`rank is not a total order — ${new Set(rankValues).size} distinct for ${GRID * GRID} nodes`);
 }
-for (let l = 0; l < anchors.length; l++) {
-  const a = anchors[l];
-  if (to[l * GRID * GRID + a] !== 0 || from[l * GRID * GRID + a] !== 0) {
-    throw new Error(`anchor ${l} (node ${a}) own cells are not 0 — table law broken`);
+if (rankValues.some((r) => !Number.isInteger(r) || r < 0 || r >= GRID * GRID)) {
+  throw new Error("a rank value is outside [0, nodeCount)");
+}
+// The shortcut band for this grid (measured 2026-10-26, this host: 41 —
+// the literature's healthy ~1.4-2.0 per node; the tripwire sits at 4x).
+const SHORTCUT_BAND_MIN = 20;
+const SHORTCUT_BAND_MAX = 80;
+if (ch.stats.shortcutCount < SHORTCUT_BAND_MIN || ch.stats.shortcutCount > SHORTCUT_BAND_MAX) {
+  throw new Error(
+    `shortcut count ${ch.stats.shortcutCount} outside the expected grid band ` +
+      `[${SHORTCUT_BAND_MIN}, ${SHORTCUT_BAND_MAX}] — check the contraction/witness budget`,
+  );
+}
+// Overlay-aware weight map: a cascading shortcut's constituents are the
+// OVERLAY edges it replaced (originals OR earlier shortcuts — emission
+// order is the contraction order, so replaying rank[via]-ascending
+// rebuilds every weight exactly).
+const weight = new Map(); // "a,b" -> min overlay weight
+for (const e of edges) {
+  if ((e.flags & 1) !== 0) continue;
+  const key = `${e.from},${e.to}`;
+  const prev = weight.get(key);
+  if (prev === undefined || e.seconds < prev) weight.set(key, e.seconds);
+}
+const shortcutEntries = [];
+for (let u = 0; u < GRID * GRID; u++) {
+  for (let s = ch.shortcutStart[u]; s < ch.shortcutStart[u + 1]; s++) {
+    shortcutEntries.push({ from: u, to: ch.shortcutTarget[s], seconds: ch.shortcutSeconds[s], via: ch.shortcutVia[s] });
   }
 }
-const coverage = landmarkCoverage(to, anchors, GRID * GRID);
-if (coverage.some((c) => c !== 1)) {
-  throw new Error(`the grid is strongly connected — coverage must be 1.0, got ${coverage.join(", ")}`);
+shortcutEntries.sort((a, b) => ch.rank[a.via] - ch.rank[b.via] || a.from - b.from || a.to - b.to);
+for (const sc of shortcutEntries) {
+  const via = sc.via;
+  if (!Number.isInteger(via) || via < 0 || via >= GRID * GRID) {
+    throw new Error(`shortcut ${sc.from}->${sc.to} has an out-of-range via ${via}`);
+  }
+  if (ch.rank[sc.from] <= ch.rank[via] || ch.rank[sc.to] <= ch.rank[via]) {
+    throw new Error(`shortcut ${sc.from}->${sc.to} (via ${via}) is not upward`);
+  }
+  const w1 = weight.get(`${sc.from},${via}`);
+  const w2 = weight.get(`${via},${sc.to}`);
+  if (w1 === undefined || w2 === undefined) {
+    throw new Error(`shortcut ${sc.from}->${sc.to} (via ${via}): a constituent overlay edge is missing`);
+  }
+  if (sc.seconds !== Math.fround(w1 + w2)) {
+    throw new Error(`shortcut ${sc.from}->${sc.to} weight ${sc.seconds} != f32(${w1} + ${w2})`);
+  }
+  const key = `${sc.from},${sc.to}`;
+  const prev = weight.get(key);
+  // The contraction's overlay accumulates f64 sums (the f32 rounding
+  // happens once, at CSR assembly) — replay the same f64 domain so the
+  // next cascade level compares exactly.
+  if (prev === undefined || w1 + w2 < prev) weight.set(key, w1 + w2);
+}
+
+// Strong connectivity (the grid is bidirectional except the ONE oneway
+// mirror — every node reaches every node).
+const adj = Array.from({ length: GRID * GRID }, () => []);
+for (const e of edges) {
+  if ((e.flags & 1) !== 0) continue;
+  adj[e.from].push(e.to);
+}
+const seen = new Uint8Array(GRID * GRID);
+const stack = [0];
+seen[0] = 1;
+while (stack.length > 0) {
+  const u = stack.pop();
+  for (const v of adj[u]) if (!seen[v]) { seen[v] = 1; stack.push(v); }
+}
+if (seen.some((s) => s === 0)) {
+  throw new Error("the grid is not strongly connected — fixture invariant broken");
 }
 
 const bin = encodeGraph({
@@ -121,11 +184,18 @@ const bin = encodeGraph({
   edges,
   names,
   maxSpeedKmh: SPEED_PERIMETER_KMH,
-  landmarks: { quantumS: LANDMARK_QUANTUM_S, nodes: anchors, to, from },
+  ch: {
+    rank: ch.rank,
+    shortcutStart: ch.shortcutStart,
+    shortcutTarget: ch.shortcutTarget,
+    shortcutSeconds: ch.shortcutSeconds,
+    shortcutVia: ch.shortcutVia,
+  },
 });
 writeFileSync(new URL("./mini-graph.bin", import.meta.url), Buffer.from(bin));
 console.log(
   `mini-graph.bin: ${GRID * GRID} nodes, ${edges.length} edges, 3 names, ` +
-    `L=${LANDMARK_COUNT} landmarks (nodes ${[...anchors].join(",")}), quantum ${LANDMARK_QUANTUM_S}s, ` +
-    `coverage ${coverage.map((c) => c.toFixed(2)).join("/")}, ${bin.byteLength} B`,
+    `CH shortcuts ${ch.stats.shortcutCount} (${ch.stats.ratioPerNode.toFixed(2)}/node, ` +
+    `witness searches ${ch.stats.witnessSearches}, budget hits ${ch.stats.witnessBudgetHits}), ` +
+    `${bin.byteLength} B`,
 );