feat(08-02): ARG2 codec — one version, one decode path (G-suite GREEN)

graph.ts versioned IN PLACE (no second codec): GRAPH_MAGIC 0x41524732,
GRAPH_VERSION 2, HEADER_SIZE 72 (72%8==0 keeps f64 views aligned);
header gains landmarkCount u32 @60 (the old pad), landmarkQuantumS f32
@64, reserved u32 @68; the landmark suffix (u16 toTable, u16 fromTable,
u32 landmarkNodes) extends layoutFor's total arithmetic — exact-size
law covers the variable suffix BEFORE any view allocation (T-08-01).
D-01 fail-honest: ARG1 bytes / version!=2 / landmarkCount=0 / hostile
quantum / size mismatch all decode null (committed mini-graph-arg1.bin
is the standing rejection vector). GraphInput.landmarks REQUIRED and
loud-validated (T-08-04). RoadGraph gains the five landmark fields
(Plan 08-03's ALT potentials consume them).

Fixture regenerated as ARG2 through the SAME encode authority: the
builder gains the tiny real landmark stage (L=2, anchors [0,18],
quantum 0.1s, coverage 1.0 — identical bytes every re-run).

Rule-3 call-site updates forced by the required-landmarks law (missed
by the plan's files list): astar.test A3 + snap.test S3 synthetics
carry inert hand landmark blocks. Fixture regen pulled forward from
task 3 so this commit's own battery is green (74 passed, 0 failed).

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

@@ -16,6 +16,12 @@
  * 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.
+ *
  * 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):
  *   - 25 nodes, 80 directed edge entries (20 H + 20 V segments, both ways)
@@ -28,6 +34,7 @@
 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";
 
 const GRID = 5;
 const LAT0 = 3.15;
@@ -85,6 +92,40 @@ reverse.flags = 1;
 
 if (edges.length !== 80) throw new Error(`expected 80 directed edges, built ${edges.length}`);
 
-const bin = encodeGraph({ lat, lng, edges, names, maxSpeedKmh: SPEED_PERIMETER_KMH });
+// 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");
+}
+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`);
+  }
+}
+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(", ")}`);
+}
+
+const bin = encodeGraph({
+  lat,
+  lng,
+  edges,
+  names,
+  maxSpeedKmh: SPEED_PERIMETER_KMH,
+  landmarks: { quantumS: LANDMARK_QUANTUM_S, nodes: anchors, to, from },
+});
 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, ${bin.byteLength} B`);
+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`,
+);