F14.1b — Generate a bounded palette and numbered guide buffer

[Flow-Fly]

Parent: #129. Depends on the sampled grid from the previous F14.1 child.

Scope

Convert sampled grid pixels into a deterministic generated palette and a one-based numbered guide buffer.

Implementation

• Add a pure palette-reduction module under src/services/paint-by-number/.
• Accept sampled ImageData plus a maximum generated color count.
• Use a deterministic bounded quantization strategy suitable for small grids.
• Map transparent cells to index 0.
• Map opaque cells to palette indices 1..N using one consistent perceptual color-distance function.
• Return { palette, target, width, height } with a compact typed target buffer.
• Do not reuse extractColorsFromDrawing as if it were already a bounded quantizer; share only genuinely reusable color helpers.

Acceptance

• The palette never exceeds the requested color count.
• Every nonzero target index resolves to an existing palette entry.
• Transparent cells remain 0.
• Fixed fixtures produce stable palette and target hashes.
• No project state or UI is introduced.

Verification

• Targeted Vitest coverage
• npm run lint
• git diff --check

[Flow-Fly]

Completed in PR #310. The implementation is merged into develop at b3bf845 and the post-merge CI run 29084411483 passed.