Extract now-playing playback lifecycle

[thewrz]

Summary

• Move now-playing playhead, display-progress, cache-key sync, and waveform envelope ownership into NowPlaying.
• Delegate playback start/tick/clear and view-time envelope lookup from the app to the owner.
• Keep AudioStore focused on decoded PCM and add owner-level lifecycle tests.

Design decisions

• Kept playing in HonkHonk for tile highlighting and existing app flow; NowPlaying keeps its own active id only to maintain the waveform cache key.
• NowPlaying::clear() resets active UI state but preserves per-sound envelope cache entries, matching the prior decode-cache behavior and keeping envelopes pre-volume.
• Added PlaybackStart to group decoded PCM metadata and stay within the repo's strict clippy argument limit.

Validation

• cargo test
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings

Closes #142

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 56d34102-854f-434b-add2-dd5636861caa

📥 Commits

Reviewing files that changed from the base of the PR and between 9d493cb and c8af094.

📒 Files selected for processing (5)

• src/app/mod.rs
• src/app/playback.rs
• src/audio/store.rs
• src/ui/now_playing.rs
• src/ui/now_playing/tests.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/issue-142

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[thewrz]

Review loop transparency note: CodeRabbit did not run a real review on this cycle because it reported review limits / usage credits exhausted. Claude ultrareview was not run from this environment because the sandbox reviewer blocked the third-party cloud review call as potential off-platform code exfiltration. I manually reviewed the PR diff for src/app/mod.rs, src/app/playback.rs, src/audio/store.rs, src/ui/now_playing.rs, and src/ui/now_playing/tests.rs; found no actionable findings; and verified locally with cargo test (492 lib tests, 5 main tests, 19 decoder tests, plus zero-test integration/doc targets all passed). CI is green at head 519f345.

[thewrz]

Claude ultrareview backup for the CodeRabbit usage block has been handled.

Addressed in c8af094:

• Fixed the cold-play stale waveform window by marking the now-playing state as pending before async decode completes.
• Reworked NowPlaying::sync_active to avoid cloning the active id on every frame.
• Added regression coverage for the stale waveform cache-key case.

Local verification run after the fix:

• git diff --check
• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings