feat(06-01): typed-array A* + turn classifier + step derivation + public exports (GREEN)

- routeAstar: allocation-free hot loop — binary heap of node indices over
  preallocated typed arrays (grown by doubling), typed-array g/closed/
  came-from, on-demand cached haversine/maxSpeed heuristic (admissible +
  consistent); oneway-reverse edges never relaxed; unreachable -> null
- A1 proves A* == brute-force Dijkstra on ALL 625 mini-graph pairs
- classifyTurn: the one turn authority (compass convention — negative
  delta = left; plan's (0,40)->slight-left case contradicted its own
  (0,-90)/(0,90) pins, documented deviation); TURN_* constants exported
  for plan 07's cron projection
- deriveSteps: splits at non-straight bearings or name changes; steps
  carry kind/name/distanceM + polyline6; depart-first, arrive-last
- index.ts: engine-A named exports + ladder note updated — backend #2 is
  now 'engine-a' (D-01 supersedes the wasm-router note; Valhalla port
  infeasible + geojson-path-finder rejected, engine B = post-v1
  evolution, D-02); createHttpRoutingSource deliberately absent (plan 04)

routing/src/index.ts

@@ -4,12 +4,28 @@
  * Named exports ONLY (no default export). The package has NO exports /
  * main field (the @araharah/theme + @araharah/search precedent), so
  * consumers import the EXPLICIT SUBPATH WITH EXTENSION —
- *   import { createFixtureRoutingSource } from "@araharah/routing/src/index.ts";
+ *   import { snapToGraph } from "@araharah/routing/src/index.ts";
  * (both tsconfigs carry allowImportingTsExtensions + bundler resolution;
  * esbuild and Vite compile the .ts source; Node 24 type-stripping handles
  * the corpus runner's relative form). A bare "@araharah/routing" specifier
  * resolves to NOTHING — never use it.
  *
+ * THE BACKEND LADDER (D-01 supersedes the phase-5 "wasm-router" note):
+ *   #1  "fixture"   — createFixtureRoutingSource: the golden-pairs demo
+ *                     corpus (15 committed KL pairs).
+ *   #2  "engine-a"  — the Worker-side router built on THIS plan's data
+ *                     plane: decodeGraph + snapToGraph + routeAstar over an
+ *                     R2-hosted CSR graph binary. Engine B (a fully
+ *                     client-side webview router — graph on the phone) is
+ *                     engine A's documented post-v1 evolution (D-02). The
+ *                     Valhalla-to-JS port was ruled infeasible and
+ *                     geojson-path-finder rejected (demo-grade: no
+ *                     alternatives/one-ways) — recorded so neither is ever
+ *                     re-proposed (D-02).
+ *   Plan 04 adds `createHttpRoutingSource` (the /api/route surface that
+ *   plugs engine A beneath the RoutingSource seam) — deliberately absent
+ *   here.
+ *
  * The module reads NO environment, NO secrets, and NO network (D-05/D-12):
  * the whole routing stack is V8-native and dependency-free — fixtures ride
  * the bundle, requests never leave the caller's frame.
@@ -28,3 +44,22 @@ export {
   getPairOrigin,
   getPairs,
 } from "./fixture-source.ts";
+export type { RoadGraph, GraphEdgeInput, GraphInput } from "./graph.ts";
+export {
+  decodeGraph,
+  encodeGraph,
+  GRAPH_MAGIC,
+  GRAPH_VERSION,
+  UNNAMED_NAME_ID,
+} from "./graph.ts";
+export type { SnapResult } from "./snap.ts";
+export { snapToGraph, SNAP_BBOX_MARGIN_DEG } from "./snap.ts";
+export type { AstarResult, GuidanceTurnKind } from "./astar.ts";
+export {
+  routeAstar,
+  deriveSteps,
+  classifyTurn,
+  TURN_STRAIGHT_MAX_DEG,
+  TURN_SLIGHT_MAX_DEG,
+  TURN_UTURN_MIN_DEG,
+} from "./astar.ts";