Skip pitch-shifting for percussive types, add result indexing

[LukeMainwaring]

Summary

• Unpitched types: Define canonical UNPITCHED_TYPES set in sample_type.py (kick, snare, hihat, clap, cymbal, percussion, drum, fx). Pitch-shifting is skipped for these types across transform tools, kit builder, and pair scoring — BPM time-stretching still applies. Consolidates and replaces the old ONE_SHOT_TYPES in candidate search.
• Result indexing: Search results now include a 1-based index field, rendered as a numbered badge on sample cards. Agent instructions updated to resolve ordinal references ("the 3rd one", "Add wavesurfer.js waveform visualization to sample browser #2") from these indices.
• Demo workflow docs: Rewrote DEMO_WORKFLOWS.md to use natural references (chore: bump all dependencies to latest versions #1, Add wavesurfer.js waveform visualization to sample browser #2) instead of [sample_id] placeholders, and documented the unpitched pitch-shift skip behavior.

Changes

• backend/src/samplespace/schemas/sample_type.py — Added UNPITCHED_TYPES set
• backend/src/samplespace/agents/tools/transform_tools.py — Skip pitch-shift for unpitched types
• backend/src/samplespace/services/candidate_search.py — Replace ONE_SHOT_TYPES with UNPITCHED_TYPES
• backend/src/samplespace/services/kit_builder.py — Gate key compat scoring on pitched types
• backend/src/samplespace/services/pair_scoring.py — Gate key scoring on pitched types
• backend/src/samplespace/agents/tools/formatting.py — Add index to sample payloads
• backend/src/samplespace/agents/sample_agent.py — Agent instructions for ordinal resolution
• frontend/components/elements/sample-card.tsx — Render index badge
• README.md, docs/DEMO_WORKFLOWS.md, docs/preference-learning-flow.md — Doc updates

Test Plan

• Build a kit with song context set — verify drum/percussion slots are NOT pitch-shifted but ARE BPM-stretched
• Run match_to_context on a drum loop — confirm "Pitch-shift skipped" message, BPM stretch still applies
• Run match_to_context on a tonal loop (bass, pad) — confirm pitch-shifting still works normally
• Search for samples — verify results show numbered indices (chore: bump all dependencies to latest versions #1, Add wavesurfer.js waveform visualization to sample browser #2, Add one-shot vs loop sample classification #3...)
• Reference a result by number in chat ("find more like Add wavesurfer.js waveform visualization to sample browser #2") — verify agent resolves correctly
• uv run --directory backend pre-commit run --all-files passes
• pnpm -C frontend lint passes

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