feat(avatar): import plain PNG pixel art and derive parts from others' parts

[DoodleBears]

Closes #1219

Stacked on #1246 (#1217). Merge #1246 first, then retarget this PR to main.

What changes

• PNG import. You can import a plain PNG that is the size of a part slot, or a whole-number multiple of it (up to ×16, 1 MB, 256 colors).
  • You see the image's size and color count first, then choose how many colors to keep. Each color snaps to the nearest kept color, with no dithering.
  • Replace color maps the imported fixed colors onto appearance colors and tones.
  • The PNG reader is in packages/core/src/bots/part-image.ts, and the bridge has a new partLibraryImportImage method.
• Built-in headpieces. Opening one gives you a flattened copy to start drawing from.
• Derived parts. A part saved from a library part records its parent and keeps the original author as parentAuthor.
• Library filter. Two new origins: derived and imported-image.

Evidence

Screenshots and a recording of the flow are in docs/assets/pr/1219-png-import/.

Checks

• Before the rebase onto feat(avatar): color every hair piece and wear built-in headpieces beside an accessory #1246's head, I ran tsc, lint, format and tests locally, including the new part-image.test.ts. They passed, except client-observer.test.ts, which already fails locally on node:sqlite bundling.
• The rebase applied cleanly, and I have not re-run the checks since. This PR's CI will run them.

Human QA

Please check the import flow in the recording.

🤖 Generated with Claude Code

https://claude.ai/code/session_01MozvQ2eMvqX1MzSqm9G2o8

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📥 Commits

Reviewing files that changed from the base of the PR and between 69c366c and aaa76b8.

⛔ Files ignored due to path filters (3)

• docs/assets/pr/1219-png-import/png-import-flow.gif is excluded by !**/*.gif
• docs/assets/pr/1219-png-import/png-import-flow.mp4 is excluded by !**/*.mp4
• docs/assets/pr/1219-png-import/source-tiara.png is excluded by !**/*.png

📒 Files selected for processing (30)

• CHANGELOG.md
• CHANGELOG.zh.md
• docs/architecture/botharness-architecture.en.md
• docs/architecture/botharness-architecture.md
• docs/assets/pr/1219-png-import/README.md
• docs/assets/pr/1219-png-import/builtin-flatten-en-light.webp
• docs/assets/pr/1219-png-import/derived-en-light.webp
• docs/assets/pr/1219-png-import/image-count-en-light.webp
• docs/assets/pr/1219-png-import/image-recolor-en-light.webp
• docs/assets/pr/1219-png-import/image-reduce-en-light.webp
• docs/assets/pr/1219-png-import/image-remap-en-light.webp
• docs/assets/pr/1219-png-import/image-worn-en-light.webp
• packages/client/src/client/actions.ts
• packages/client/src/client/avatar-appearance-editor.tsx
• packages/client/src/client/bridge.ts
• packages/client/src/client/custom-part-editor.tsx
• packages/client/src/client/locale.ts
• packages/client/src/client/profile-header.tsx
• packages/client/src/client/styles.ts
• packages/client/test/avatar-appearance-editor.test.ts
• packages/client/test/bot-sidebar.test.ts
• packages/client/test/custom-part-tools.test.ts
• packages/core/src/bots/avatar-appearance.ts
• packages/core/src/bots/part-image.ts
• packages/core/src/bots/part-library.ts
• packages/core/src/bridge/methods.ts
• packages/core/src/bridge/rpc.ts
• packages/core/test/bridge-rpc.test.ts
• packages/core/test/part-image.test.ts
• packages/core/test/plugin.test.ts

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