[khiet]

Summary

Monarch's road narrows toward Monaco's own at the hairpin and Rascasse. The sim reads only the sidecar and the glTF, so only the track generator changed.

Layout.half_width_at(s)
  inside a Taper  -> linear from the width before it to the width after it
  else            -> half_widths[segment] or asphalt_half_width

Monarch (full width)	before	after
Mirabeau, kink, hairpin approach and exit, Rascasse	12 m	9 m
Grand Hotel hairpin	12 m	8 m
Mirabeau Bas (the bot gate widened it from 9 m)	12 m	10 m
Everywhere else	12 m	12 m

Every taper is linear, so the edge never steps:

• In over the last 15 m of the Casino descent, after the wheel crossing.
• Out over Portier, so the road is 12 m from the start of the tunnel entry.
• In over the last 15 m of the quay, after its box row.
• Out down the Rascasse exit straight, back to 12 m about 30 m before the pit lane's fork.

These all follow the local width: the asphalt, sand, walls (including the pit lane side's), painted kerbs, checkpoints, hoardings, gantry, start line and wheel crossing. A tunnel, bridge, ramp, pit lane or hazard zone needs one width along its whole length, so the export refuses a layout whose width changes under one (refuse_width_changes).

ADR 0106 supersedes ADR 0064's uniform 12 m. The style guide's wall entry and the tracks README are updated.

Evidence

• Before: at the issue's starting widths (Mirabeau Bas at 9 m), the bot gate in_200_races_of_eight_bots_on_monarch_every_one_finishes_at_every_level failed:

  Basic seed 165: karts [3] never finished
  

  A boosted bot cleared the Mirabeau Bas wall and flew past Portier's checkpoint outside its reach. It was the only failure in 600 races.
  After: with Mirabeau Bas at 10 m (one 0.5 m half-width step), the gate passes all 600 races.
• test_road_width.py (18 tests) checks:
  • each zone's half width
  • every taper's endpoints
  • no step anywhere round the lap: the steepest change is 0.1 m of half width per metre
  • 12 m from at least 20 m before the pit lane's fork
  • asphalt, checkpoints, kerbs and hoardings at the local width
  • every box, pad, coin and sausage kerb inside it
  • the refusals
• The wall and checkpoint tests in test_track_limits.py and test_pit_lane.py now read the local width.
• Test results:
  • pnpm test:tracks: 287 passing
  • cargo nextest run --workspace: 600 passing
  • native and wasm replay tests: passing, after re-recording monarch-lap-with-bots
  • repo-checks: 254 passing
• pnpm export:tracks changed only Monarch's glb and sidecar.

Merge Danger

Door: two-way

Reverting restores the 12 m road. The replay fixture would need re-recording again.

Blast Radius: Monarch

This changes how Monarch races: tighter corners, and the walls come closer at Mirabeau and Rascasse. Bot finishing times on Monarch will shift. Other courses export unchanged.