feat(coverage): --coverage-diff to report only lines changed since a ref

[Chemaclass]

🤔 Background

Related #1032

The coverage report was whole-file. For a pull request the useful question is are the lines I touched covered — and a whole-file percentage moves for reasons unrelated to the change: adding a well-covered file raises the total while saying nothing about the new code, and the reverse hides untested additions behind a healthy number.

💡 Changes

• --coverage-diff <ref> restricts the console report to lines added or modified since the ref, reusing the three sources git_changed_files already merges (commit range, working tree, untracked counted in full)
• Pure deletions contribute nothing, and changed lines that are not executable are ignored, so a comment-only commit is not penalised
• With --coverage-min the gate follows the report — a change that fully covers itself passes inside a poorly covered file. A change with no executable lines scores 100%, not 0%
• LCOV and HTML stay whole-file; genhtml/Codecov do their own diffing and expect complete records

Decisions worth flagging

The base ref is required, not defaulted. An optional value would make --coverage-diff tests/ silently swallow the path as a ref.

A missing repo, missing git, or unresolvable ref is a hard error. On a shallow CI clone the base is simply absent — left unchecked that reports "no changed lines" and passes a diff threshold while measuring nothing, which is a silent false pass rather than a missing feature.

Verification

Mutation-tested: counting non-executable lines fails 2 tests, treating every changed line as hit fails 3, including deletion-only hunks fails 4. 26 tests green on the Bash 3.0 floor.

⚠️ Pre-existing failure on main, not from this PR

test_built_binary_stays_below_500_kib is already red on main: 513,422 bytes vs a 512,000 budget, before any change of mine (this PR adds ~5 KiB on top). It passes in CI only because the test skips when shfmt/jq are unavailable there, so it went unnoticed through the recent merges. Worth its own issue.