Clarify homepage progress scope and proof completion

[Deicyde]

Summary

• relabel the homepage metric as progress within the scoped roadmap and count completion from each target's proof semantics rather than its display/readiness state
• show the author-declared source coverage dispositions beside the metric and link directly to the coverage contract
• reserve 100% for actually complete target sets while preserving completed definitions and verified Mathlib targets
• document the distinction for human reviewers and cover blocked/ready statements, Mathlib targets, rounding, empty/singular roadmaps, and the bundled Cabannes example

Testing

• make lint
• make test (605 passed)
• make check-example
• skill-creator validation for skills/human-review

No Lean declarations changed, so lake build is not required.

Closes #27