docs: plan tempo, arrangement, and sampler tasks

[britle20]

Summary

References #23
References #25
References #26

Plans the next implementation issues for the active workflow. Tempo control remains first because BPM affects all scheduled drum and note playback. Arrangement view UI shell is placed before sampler sustain because it is a scoped UI mode-switch task and does not require arrangement data or playback.

Changes

• Adds docs/features/09-sampler-advanced-sustain.md for reusable sample-based sustain planning.
• Adds docs/features/10-tempo-control-and-bpm-slider.md for making the BPM slider control real playback tempo.
• Adds docs/features/11-arrangement-view-ui-shell.md for rendering a SONG mode arrangement UI shell from the Stitch visual reference.
• Updates PLANS.md Active Issue Order to run Tempo control and live BPM updates #25, then Arrangement view UI shell #26, then Sampler for advanced sustain #23.
• Separates milestone wording for Iowa Piano one-shot playback, tempo control, arrangement UI shell, and advanced sampler sustain.
• Extends docs/audio-engine.md with generic sampler sustain responsibilities and tempo-control rules.
• Extends docs/data-model.md with planned sampler metadata and serializable tempoBpm guidance.
• Extends docs/ui-design.md with PAT/SONG workspace mode behavior and arrangement layout rules.
• Updates docs/testing.md with sampler sustain and BPM-change checks.
• Marks feature 08 as done after PR feat: add pitched instrument selection #22 merged.

Testing

• git diff --check
• npm run typecheck --if-present
• npm run lint
• npm run test
• npm run build

Screenshots

Not applicable. Documentation-only planning PR.

Notes / Limitations

• This PR does not implement tempo control, arrangement UI, or sampler playback changes.
• Tempo control should run before arrangement UI shell and sampler sustain in $process-task.
• Arrangement view planning treats the provided Google Stitch/Tailwind HTML as visual reference only; implementation must use React, TypeScript, CSS Modules, and semantic tokens.
• Arrangement UI shell excludes real arrangement data model changes, persisted clip instances, arrangement playback, and drag/drop editing.
• Sampler planning avoids automatic loop detection and visual loop editing for now.

Follow-up

• After this planning PR merges, $process-task should implement Tempo control and live BPM updates #25 from docs/features/10-tempo-control-and-bpm-slider.md.
• Then continue to Arrangement view UI shell #26 from docs/features/11-arrangement-view-ui-shell.md.
• Then continue to Sampler for advanced sustain #23 from docs/features/09-sampler-advanced-sustain.md.