feat(mixer): add AFL on every aux master and group bus

[srperens]

Summary

• Adds per-bus AFL (After-Fader Listen) on every aux master and group, sharing the existing channel PFL/AFL solo bus and monitor-source gating
• Backend taps aux{N}_out_tee / group{K}_out_tee post-master, post-mute through a volume gate + queue into solo_mixer; new auxN_afl / groupN_afl Bool properties are live: true, persist: Some(false) and use the existing bool_to_volume transform
• is_solo_property_name (renamed from parse_solo_property_name) now matches aux/group AFL so the state layer's solo-intent cache and monitor-gate refresh handle bus AFL with the same single-batch atomicity as channel solos
• Frontend adds an AFL button under Mute on each aux/group strip, hydrates/saves/resets the new state, and extends the Clear-all batched PATCH to release bus AFLs in the same call so the monitor gates ramp back to Main exactly once
• PFL on buses is intentionally not exposed (bus faders are essentially always up); the *_afl namespace leaves room for *_pfl later without a breaking change

Test plan

• cargo check clean on workspace
• cargo test --lib mixer — 88 passed (including updated test_mixer_pfl_afl_are_transient covering the new aux/group AFL props and rewritten test_is_solo_property_name_matches_pfl_and_afl)
• cargo fmt --all + clippy clean (pre-commit hook)
• Manually verify in running mixer: toggle aux AFL → bus appears on Monitor; toggle group AFL → same; Clear-all releases all channel + aux + group solos in one ramp
• Confirm monitor strip MAIN/SOLO indicator flips when only a bus AFL is engaged (no channel solo)

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