feat(state): persist assigned tile graphics

[thewrz]

Why

Per-sound tile graphics need a safe persistence boundary before file selection, copying, or rendering can be built. Existing sound metadata must migrate without losing customizations, and invalid paths must never cause HonkHonk to overwrite unreadable state.

What

Adds a validated application-owned graphic filename to per-sound metadata, advances the persisted envelope to v2 with explicit v1 and legacy migration, and preserves default-entry pruning. Invalid graphic references and unsupported versions load read-protected. This remains state-only: no chooser, copying, decoding, thumbnails, or rendering.

Testing

• Focused sound metadata tests pass
• Full cargo test passes (686 tests)
• cargo clippy -- -D warnings passes
• cargo fmt --all -- --check passes
• CI green

🤖 Co-authored by Codex. Closes #206.

Summary by CodeRabbit

• New Features

  • Added support for assigning, updating, and clearing a graphic for each sound.
  • Added validation to prevent unsafe or invalid graphic filenames.
  • Added compatibility for loading existing metadata while preserving assigned graphics.

• Bug Fixes

  • Saving sound metadata now preserves previously stored settings, including favorites.

• Data & Reliability

  • Improved metadata persistence, migration, validation, and protection against overwriting invalid or unsupported data.

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds validated per-sound graphic references, store APIs, v1-to-v2 metadata migration, read-protected persistence behavior, application field preservation, and comprehensive tests for validation, pruning, round trips, and compatibility.

Changes

Assigned graphic persistence

Layer / File(s)	Summary
Graphic metadata contract and store APIs docs/superpowers/specs/..., src/state/sound_meta.rs, src/state/mod.rs, src/app/mod.rs, src/state/sound_meta/tests.rs	Adds GraphicAssetRef validation, SoundMeta.assigned_graphic, assignment accessors, default pruning, public exports, field-preserving save updates, and tests for metadata behavior.
Versioned persistence and migration src/state/sound_meta/persistence.rs, src/state/sound_meta/tests.rs, docs/superpowers/specs/...	Migrates version 1 data into the current model, handles current envelopes, and tests round trips, legacy shapes, unsupported versions, invalid references, and overwrite protection.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant SoundMetaStore
  participant PersistenceDecoder
  participant GraphicAssetRef
  SoundMetaStore->>PersistenceDecoder: load versioned metadata
  PersistenceDecoder->>GraphicAssetRef: validate persisted filename
  GraphicAssetRef-->>PersistenceDecoder: validated reference or error
  PersistenceDecoder-->>SoundMetaStore: migrated or current metadata

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Possibly related PRs

• wrzonance/HonkHonk#102: Updates the same per-sound metadata and save flow.
• wrzonance/HonkHonk#203: Changes the same versioned sound metadata persistence layer.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The PR implements the linked issue scope with a validated graphic reference, backward-compatible migration, pruning, and tests.
Out of Scope Changes check	✅ Passed	The changes stay within state persistence and supporting tests with no unrelated feature work.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: persisting assigned tile graphics in state.

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[thewrz]

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