feat(06-04): road-graph build tool — Overpass extract → pinned CSR binary with the D-01 stats gate

- maintainer-time repo tool (nothing in bot/web/routing src imports it): drivable-class
  Overpass extract (Pitfall 1 filter), nodes deduped by rounded coord, oneway/-1/roundabout
  flag law, maxspeed-tag overrides over class defaults, name-pool interning
- bbox derived mechanically from the committed corpus (gazetteer entries + golden pairs
  ± 0.05 deg, printed) — Open Q 4's pinning
- collect-then-write + ENFORCED size gate (>1 MB, <=80 MB, exit 1 outside) + advisory
  30-80 MB band warning; --check mode decodes and re-prints the gate, zero network
- routing/data/.gitignore: *.bin is an ODbL-derived build artifact — forks build their own
- real KL graph built: 277514 nodes / 598396 edges / 21447 names / 13.1 MB (gate PASS,
  band warning printed for review)

routing/data/.gitignore

@@ -0,0 +1,3 @@
+# D-01: the road-graph binary is an ODbL-derived BUILD ARTIFACT, never
+# committed — forks build + upload their own (see docs/ROAD-GRAPH.md).
+*.bin