[A-EDev]

Summary

Part 7 of 22 of the music player plan (Android parity). Stacked on feat/music-equalizer (PR #183); review and merge in order.

• Music speed 0.5x to 2x with the pitch kept (music_playback_speed); on Linux the rate is written only when it changes.
• The volume slider follows a perceptual curve, with a one-time conversion of the saved value.
• Loudness is measured in Android's order (perceptual first) and never cuts more than 20 dB.
• Optional crossfade, 1 to 12 s, off by default; same-album pairs and repeat-one stay gapless.
• The music audio graph is built on the next play instead of at settings load.

Related issue

Change type

• Bug fix
• Feature
• Refactor or maintenance
• Documentation
• Build, packaging, or CI

Validation

• pnpm test: 509 tests passed, 0 failed
• pnpm build: passed
• cargo fmt --manifest-path src-tauri/Cargo.toml --all -- --check: clean
• cargo clippy --manifest-path src-tauri/Cargo.toml --all-targets --all-features: no new warnings against main (compared at the top of the stack)
• cargo test --manifest-path src-tauri/Cargo.toml --all-targets --all-features: 784 passed, 0 failed (all sync_* suites included)
• I smoke-tested the affected desktop behavior. macOS, combined build: speed 1.5 reaches the element with pitch kept.
• OS/device: device testing by the owner. The checks above ran on macOS 27 (Apple Silicon MacBook Pro, 120 Hz) in a dev build of the whole stack; Windows and Linux were not run.

Screenshots or recordings

Risk and compatibility

• No new secrets, private data, telemetry, or broad Tauri permissions were introduced.
• User-facing documentation was updated where needed (changelog entry in v0.1.0-beta3.txt).
• Dependency and lockfile changes are intentional and limited to this PR. No dependency changes.
• Breaking changes and upgrade steps are clearly documented. No breaking changes.

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