feat(05-02): pinned golden-pair corpus + OSRM-demo fetch tool + populated committed dataset (GREEN)

- routing/test/fixtures/golden-pairs.json: the hand-authored 15-pair pinned
  corpus (D-02/D-12) — origins AND destinations are gazetteer landmarks,
  klBbox + 100 KB gz budget + probe id + alternatives floor pinned
- routing/scripts/fetch-route-fixtures.mjs: the maintainer build tool
  (overpass-topup.mjs pattern) — D-12 boundary in the header, pre-flight
  public-coordinate proof against the gazetteer (1e-4 deg), mandatory
  User-Agent, 1200 ms inter-request delay, one retry on network/5xx,
  collect-then-write, --only=<pairId> single-pair repair merge
- routing/src/data/route-fixtures.json: 15 real OSRM-demo pairs committed
  (1-3 verbatim polyline6 routes each; 8 pairs carry >= 2 alternates),
  ODbL header fields preserved (T-05-06)
- loader-env L3 armed (RED commit 01b0ed6) + C6 re-pinned from the empty-seed
  state to the populated corpus (05-01 documented both as 05-02 handoffs)
- npm delegates: root routing:fixtures; routing fixtures + corpus scripts

package.json

@@ -30,6 +30,7 @@
     "routing:test": "npm test -w routing",
     "routing:typecheck": "npm run typecheck -w routing",
     "routing:corpus": "node routing/scripts/route-corpus.mjs",
+    "routing:fixtures": "node routing/scripts/fetch-route-fixtures.mjs",
     "web:dev": "npm run dev -w web",
     "web:build": "npm run build -w theme && npm run build -w web",
     "web:build:harness": "npm run build:harness -w web",