fix(driving): correct planned-stop exit warnings and stuck ramps

[ironcross32]

What changed and why

The "You drove past your planned stop" warning was decided purely from the highway odometer in Trip._check_stops, with no awareness of the exit/ramp/signal state. So signaling a planned stop and braking down the exit ramp toward it triggered a false "plan cancelled" the moment the mile marker slipped behind, even though the driver was stopping there.

While investigating, we also discovered there was no invalidation of a stop when the driver signaled and took the ramp but never actually stopped: the ramp waited indefinitely, speed enforcement stayed off, the highway odometer kept advancing, and stopping miles later still opened that stop's menu. That gap predates the planned-stop work — it came in with the original highway-exits feature — and has now been corrected.

The fix:

• Trip._check_stops suppresses the warning while the planned stop's exit is signaled or on the ramp (published each tick via Trip._exit_in_progress), and otherwise warns once the exit marker is passed — the point past which the ramp is no longer takeable.
• A signaled-but-too-fast miss folds the cancellation into its single spoken line instead of emitting a second cue.
• A taken exit that never stops is declared blown once it rolls RAMP_OVERSHOOT_MI (0.5 mi) past the end of the ramp, cancelling the plan and handing normal highway behaviour back (enforcement, lane physics).
• While on the ramp the truck is off the highway: the mile marker now holds and the ramp consumes the movement (Trip.on_ramp / Trip.last_moved_mi) instead of the highway odometer.

What players will notice

• Signaling a planned stop and taking its exit no longer announces that you drove past it.
• If you never signal and drive past the exit, you're told once, when it's genuinely too late to take it.
• If you take an exit but never stop, the game tells you that you never stopped and returns you to the highway, rather than silently stranding you on an endless ramp.
• Your route progress no longer ticks up while you brake down an exit ramp; the highway resumes where you left it.

Tests run

• uv run pytest (full suite) — green.
• Added test_plan_survives_while_descending_ramp, test_too_fast_miss_cancels_plan_once, test_taken_exit_blown_when_never_stopping, and test_highway_odometer_holds_on_the_ramp in tests/test_stop_detail.py.
• uv run ruff check src tests — clean.
• Manual testing by the author checks out.

Accessibility impact

All changes are to spoken cues. The false cancellation is gone; the real cases each speak a single, clear line ("...drove past your planned stop. Plan cancelled." / "...too fast for the ramp and missed... Plan cancelled." / "You never stopped and drove past... Plan cancelled."), with terse-speech variants preserved. No information moved to a visual-only cue.

Changelog

• Player-facing entries added under ## Unreleased → Fixed in CHANGELOG.md.

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