perf: reduce GC scan span and eliminate padding in hot structs

[jeduden]

Summary

Addresses struct layout violations identified by auditing against docs/development/high-performance-go.md. Each fix reduces GC pointer-scan span or eliminates alignment padding in structs that are allocated per-lint-call, reducing GC pressure on the hot path. All fixes follow Red/Green TDD: failing layout/alloc test → struct reorder or function rewrite → passing test → commit.

Fixes

• tablefmt.stripPrefix — zero-alloc rewrite: The previous implementation called string(line) and []byte(…), causing 2 heap allocations per table row. Replaced with a string(line[:plen]) == prefix slice comparison the compiler optimises to a zero-alloc memcmp. Guarded by TestStripPrefixNoAlloc (skipped under -race where instrumentation inflates alloc counts).

• listscan structs — padding elimination: Reordered Item (40→32 bytes), List (72→64 bytes), frame (56→48 bytes), and markerInfo (40→24 bytes) to group bools together at the end, eliminating bool-induced alignment padding. Asserted by TestStructLayout via unsafe.Sizeof.

• crossfilereferenceintegrity structs — GC span + padding: Reordered Rule (128→120 bytes, bools moved last), checkCtx (GC span 72→56, maps before strings), wikilinkResolver (GC span 64→56, interface+pointer before strings), and targetFile (GC span 40→32, func field first). Asserted by TestStructLayout via unsafe.Sizeof.

• tableformat.Rule — GC scan span: Moved Style string first so the GC pointer-scan span is 8 bytes (string data pointer at offset 0) rather than 24 bytes (string after two int fields). Asserted by TestRuleFieldOrder via unsafe.Offsetof.

• toc.Rule — GC scan span: Moved engine *gensection.Engine before engineOnce sync.Once so the GC pointer-scan span is 8 bytes (pointer at offset 0) rather than 24 bytes (pointer buried after the 12-byte sync.Once padded to 16 for alignment). Asserted by TestRuleFieldOrder via unsafe.Offsetof.

Post-review fixes (3 rounds of /code-review --effort xhigh)

• Added raceEnabled guard to tablefmt/layout_test.go — testing.AllocsPerRun returns inflated counts under -race due to instrumentation overhead; existing tablefmt tests use the same guard pattern.
• Corrected sync.Once size in toc/layout_test.go comment: sync.Once is 12 bytes in Go 1.25, padded to 16 to align the subsequent pointer, giving a 24-byte scan span in the old layout.
• Fixed stale present-tense comment in crossfilereferenceintegrity/layout_test.go that described the struct as "currently packs bool fields" after the reorder was already applied.

Test plan

• go test ./internal/rules/listscan/... — TestStructLayout passes at 32/64/48/24 bytes
• go test ./internal/rules/tablefmt/... — TestStripPrefixNoAlloc allocates 0
• go test ./internal/rules/crossfilereferenceintegrity/... — TestStructLayout passes at 120 bytes
• go test ./internal/rules/tableformat/... — TestRuleFieldOrder passes at offset 0
• go test ./internal/rules/toc/... — TestRuleFieldOrder passes at offset 0
• go test ./... — full suite green

Reference: docs/development/high-performance-go.md

🤖 Generated with Claude Code

https://claude.ai/code/session_015pKxVjp82WDkwB4MKNNKM8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.62%. Comparing base (7d37ea5) to head (e0efed0).

Additional details and impacted files

Components	Coverage Δ
Go	98.61% <100.00%> (ø)
TypeScript	99.54% <ø> (ø)

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[jeduden]

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[jeduden]

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[jeduden]

✅ Merge Queue — merged

This PR landed on main via commit 1599c9f. CI run that validated the merge.

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