feat(rules): add MDS060 occurrence rule (plan 2607022118)

[jeduden]

Summary

• Add MDS060 occurrence rule: counts tokens or a regex pattern per scope unit (paragraph, section, file) and flags when the count falls below min or exceeds max
• Fenced and indented code blocks are excluded via CollectSectionParagraphsWithText
• Opt-in rule (enabled: false by default); supports lists: wordlist integration via WordlistTarget() = "tokens"
• 32 unit tests + 5 integration fixture tests (3 bad, 2 good); all tests green

Code review fixes applied

Four confirmed bugs found during xhigh code review and fixed before committing:

• File: f.Path missing from diagnostics — diagEach/diagCombined produced diagnostics with empty File, breaking CLI output filename display and sort order; all 61 other rules set File: f.Path
• Allocation budget violation — countToken called strings.ToLower(text) once per token per paragraph; with N tokens and P paragraphs that's N×P allocations. Fixed by pre-lowercasing text once per scope unit before the token loop, keeping Check within the ≤10-alloc budget
• FindAllString allocates matched substrings — switched to FindAllStringIndex which returns positions only, eliminating one string copy per match
• min < 0 accepted silently — negative min made the lower-bound check always true; ApplySettings now returns an error for min < 0

Files changed

• internal/rules/occurrence/rule.go — rule implementation
• internal/rules/occurrence/rule_test.go — 32 unit tests
• internal/rules/MDS060-occurrence/README.md — rule documentation
• internal/rules/MDS060-occurrence/bad/ — 3 bad fixtures
• internal/rules/MDS060-occurrence/good/ — 2 good fixtures
• internal/rules/all/all.go — import registration
• internal/integration/perrule_bench_test.go — alloc ceiling entry (MDS060: 4)
• internal/integration/testdata/rule_walk_audit.json — regenerated walk audit
• internal/rulelayer/rule_walk_audit.json — copy kept in sync
• docs/research/markdownlint-coverage/README.md, internal/rules/index.md — catalog regeneration

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.70%. Comparing base (d4af5d5) to head (8f5e5c4).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

Components	Coverage Δ
Go	98.69% <100.00%> (+<0.01%)	⬆️
TypeScript	99.54% <ø> (ø)

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[jeduden]

🟢 Merge Queue — picked up

This PR is in the queue and will be batched with other queue-labelled PRs.

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[jeduden]

🔵 Merge Queue — CI running

Merged into batch branch merge-queue/batch-754-1785006212 alongside #754, #756, #762, #764. View CI run.

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[jeduden]

✅ Merge Queue — merged

This PR landed on main via commit 47ace27. CI run that validated the merge.

Next: Done — nothing more to do here.