perf: eliminate hot-path allocations in five rules

[jeduden]

Summary

Audited the codebase against docs/development/high-performance-go.md using a four-agent parallel sweep. Fixed the top 5 allocation anti-patterns found:

• include/rule.go: Hoist []byte(text) out of the inner PI-body loop. Previously the full file content was re-allocated as []byte on every line of every directive body; now converted once before the outer AST walk and passed to bytes.Contains instead of strings.Contains(string(...)).

• firstlineheading/rule.go: Remove the unconditional fmt.Sprintf that built the "missing heading" message on every file check, even for passing files (1 alloc per workspace file). Replaced with strconv.Itoa + concatenation, deferred to the error sites only.

• githooksync/rule.go: Replace strings.Contains(string(data), marker) with bytes.Contains(data, preMergeMarkerBytes). data is already []byte from os.ReadFile; the previous code allocated a copy of the entire hook file as a string. preMergeMarkerBytes is a package-level var to avoid re-converting the constant each call.

• linkvalidity/rule.go: Add a bytes.Contains('(') guard before reversedRe.FindAllSubmatchIndex. Reversed links always start with (; this guard skips the regex NFA walk on lines without that byte — the common case.

• headingincrement/rule.go: Replace fmt.Sprintf with strconv.Itoa + string concatenation in the two diagnostic message sites. fmt.Sprintf uses reflection; strconv.Itoa is ~3× faster.

Test plan

• go build ./... — clean
• go test ./... — all packages pass
• go test ./internal/integration/... — integration suite passes
• go run ./cmd/mdsmith check . — 388 files checked, 0 failures

https://claude.ai/code/session_01QLP2AGDLaojzi8TFpPDLr8

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.27%. Comparing base (37488a7) to head (e27a2f1).

Additional details and impacted files

Components	Coverage Δ
Go	97.25% <100.00%> (+<0.01%)	⬆️
TypeScript	99.54% <ø> (ø)

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