Traffic sounds follow the real vehicles, an interstate traffic bed, and three small 1.9.4 fixes

[Orinks]

Requested by Orinks · project thread

What changed and why

Before: traffic was a one-shot pass recording fired on a 6 second cooldown, so its peak rarely matched the car it stood for. Passes were dropped, and on an exit ramp the held odometer made mainline cars "pass" dead centre. Cross-traffic cues at the ramp end came from placeholder recordings with the wrong lead. Interstates had no sound of distant traffic, and the road noise sat about 15 dB under the engine.

After: each of the three nearest real vehicles gets its own steady loop. Its volume, pan and Doppler rate are set every frame from that vehicle's actual position and speed, and the loop follows the vehicle by id. On a ramp the freeway slides off to the left as you diverge. Cross traffic at the ramp terminal sweeps across in front. A separate traffic bed follows the leg's traffic density, on interstates and divided highways only. Tire noise is 6 dB louder.

How: driving_traffic_sounds.rs replaces driving_traffic_pass.rs. CrossVehicle gained a stable id. TrafficManager::freeway_presence_at drives the bed. There are new pack entries (eight vehicle loops and traffic/highway_bed), rendered with genny from sound-test/traffic_sounds.json, with levels in docs/audio-levels.md.

Also in this PR:

• The "Engine voice" and "Engine brake voice" settings are renamed "Engine sound" and "Engine brake sound" (Orinks asked for this). Setting keys are unchanged, so saves carry over.
• Bobtail to a nearby city is refused while the CDL is suspended or disqualified, from the terminal and from an already open bobtail menu. Ported from 2.0's fix(enforcement): point a pulled CDL at waiting, and refuse a bobtail #259; this closes the found-along-the-way roadmap item.
• The radio status screen names the Playlists folder's full location while no personal playlist is on the dial and streamer-safe mode is off (issue not a bug, just a suggestion.[Bug]: #289, a player who could not find how to add their own music).

What players or maintainers will notice

• Passing traffic sounds where the vehicle is: it comes up beside you, then fades ahead or behind.
• On exit ramps, the freeway fades off to the left, and cross traffic at the ramp end sweeps across in front of you.
• Interstates have a bed of distant traffic, busier where the road is busier.
• Tire noise is louder.
• The settings read "Engine sound" and "Engine brake sound".
• An owner-operator on a pulled CDL hears why they cannot bobtail, and when that ends.
• With no playlist of their own, the radio status screen tells the player where to put playlist files.

Tests and checks run

• cargo test -p ff-core -p freight-fate (ci_quick): 3058 passed. One failure, online_presence::test_failed_fallback_replacement_keeps_the_existing_identity, is environmental: the Linux container runs as root, so a read-only directory does not block the write. It skips on Windows CI.
• New states_driving_traffic_sounds tests cover pan side, fade, Doppler, ramp divergence, cross traffic and the bed only on freeways. There is a new freeway_presence_at test in ff-core, two bobtail refusal tests in states_city, and a playlist-pointer test in synth_music (89 radio and status-screen tests pass).
• cargo fmt --all --check and cargo clippy --all-targets --locked -- -D warnings are clean.
• BASS could not be fetched in this container, so the audio-device tests skipped locally. Windows CI has BASS.
• Adversarial battery: 47 scenarios, 47 clean.

Accessibility impact

Spatial audio only: no speech was added or removed for traffic. The loops do not duck speech any differently than the old pass cues did. The bobtail refusal uses the menu channel with the error tick, the same as the board's suspension refusal. The playlist pointer is one line on an information screen the player opens, and it goes quiet once a playlist loads. The renamed setting labels were checked in states_main_menu_settings. The traffic and bed levels still need Orinks's listening pass at the wheel.

Changelog

• I added a player-facing bullet under ## Unreleased in CHANGELOG.md, written in plain player language and matching the style of the existing entries.

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