feat(coverage): add format-report helper for the report buffer

Pure helper that renders intersect records into the text shown in the coverage report buffer. Takes the list of per-file plists from cj/--coverage-intersect and a scope label, returns the formatted string.

Output has three sections depending on what's present:

- "Uncovered lines" — one line per uncovered line, formatted as "<path>:<line>: uncovered" so compilation-mode's default regex picks them up for next-error navigation.
- "Not tracked" — files changed in the diff but absent from the coverage data (READMEs, test files, config).
- "Fully covered" — tracked files where every changed line is covered.

Files with empty :changed-lines (deletion-only hunks) are omitted. Summary counts cover only tracked files, so an all-README change shows "0 of 0" rather than a misleading percentage over nothing.

Tests cover Normal (partial, fully covered, mixed sections), Boundary (empty records, 100% coverage with no uncovered section, only-not-tracked case, deletion-only exclusion), and the output format that next-error relies on.

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