F14.1a — Sample an image into a deterministic target grid

[Flow-Fly]

Parent: #129. First issue in the F14.1 foundation batch.

Scope

Add the image-decoding and area-sampling boundary that turns a local source image into a small deterministic grid before palette reduction.

Implementation

• Add a focused module under src/services/paint-by-number/.
• Accept ImageData plus a target long-side size; keep file decoding in a thin browser adapter.
• Preserve source aspect ratio and produce integer width/height of at least 1.
• Average every source region that contributes to a destination cell.
• Preserve hard transparency: cells below the documented alpha threshold become transparent.
• Keep the sampling result independent from palette selection.

Acceptance

• Landscape, portrait, square, one-pixel, and partially transparent inputs produce expected dimensions and pixels.
• The same input always produces byte-identical output.
• Tests exercise non-integer downscale factors.
• No palette, project, persistence, or UI code 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.