F14.2a — Derive creative coverage and remaining guide counts

[Flow-Fly]

Parent: #130. Depends on completed F14.1 / #129.

Add a small pure guidance model that reads the durable target and the visible pixels in the fixed painting layer.

Acceptance:

• Paint coverage means any non-transparent painted cell, regardless of color correctness.
• Transparent target cells do not count.
• Return total, covered, remaining, percentage, and remaining counts per guide number.
• Off-guide colors count as covered; erase makes a cell remaining again.
• Keep calculations deterministic and test the empty and fully covered cases.

[Flow-Fly]

Completed in PR #315. The implementation is merged into develop at a9ed5f9 and post-merge CI run 29086892806 passed.