test(tar-xz): close remaining coverage gaps to 100% lines

[oorabona]

Summary

Final tar-xz coverage push : real tests for the genuinely-untested code paths + v8 ignore wraps for the defensive branches that PR-γ left. Brings tar-xz from 95.33% to 100% lines.

Tests added (13 cases in test/coverage-final.spec.ts)

Coverage target	Test coverage
create.ts:88-104 AsyncIterable source	3 cases : multi-chunk async generator, single-chunk, ArrayBuffer branch
extract.ts:70-73 zero-byte entry	2 cases : bytes() returns empty Uint8Array, second call hits cache
file.ts:223 hardlink lstat ENOENT	Forward-reference fixture : lstat ENOENT swallowed, link() ENOENT propagates
file.ts:225-229 symlink-as-source rejection	Real symlink → hardlink fixture, asserts "Refusing hardlink"
file.ts:232 symlink-ancestor rejection	Symlink dir → hasSymlinkAncestor returns true
file.ts:275 EACCES on open	POSIX-only, it.skipIf for Windows / root
tar-parser.ts:254 mid-header truncation	XZ-compressed 256-byte buffer + empty buffer variant
tar-parser.ts:338 mid-body during list() SKIP	512/1024 body bytes, no EOA
tar-parser.ts:361 mid-padding	size=300 entry, 100/212 padding bytes truncated

v8 ignore wraps applied

File:Line	Reason
extract.ts drainSkippedEntry L165-169	TAR_PARSER_INVARIANT rethrow — corrupt parser state unreachable via public API
extract.ts makeDataGen L205-210	Concurrent-iteration guard — makeTarEntryWithData calls dataPull() exactly once per entry
extract.ts extract() L299-303	Stray-chunk invariant — parseTar never emits chunk before entry
extract.ts bytes() L78-92	Overflow guard — parseTar clips chunks at bytesRemaining, so over-delivery cannot happen via extract()
file.ts openFileExclusive L343	Win32 TOCTOU retry rethrow — same race-window pattern as PR #114 hardening

Smart fixture finding

list() and extract() call streamXz() internally — raw TAR bytes passed directly fail with LZMAFormatError. All truncated-archive fixtures wrap bytes with xzSync() first. Locks the contract for future fixture writers.

Coverage delta

Metric	Before	After
tar-xz lines	95.33%	100%
tar-xz branches	89.45%	92.53%
tar-xz functions	100%	100%
create.ts lines	~95%	100%
extract.ts lines	partial	100%
tar-parser.ts lines	partial	100%
file.ts lines	96.6%	100%

Branch ceiling is 92.53% because v8 ignore start/stop suppresses body lines but the condition's branch counter still increments. Only /* v8 ignore next */ would suppress the condition, but that form is explicitly forbidden by the project convention.

Diff

3 files changed, +513 / 0 :

• packages/tar-xz/src/node/extract.ts : +8 (4 wraps)
• packages/tar-xz/src/node/file.ts : +2 (1 wrap)
• packages/tar-xz/test/coverage-final.spec.ts : NEW, +503 LOC

Gates

• pnpm install --frozen-lockfile : EXIT 0
• pnpm --filter tar-xz build : EXIT 0
• pnpm type-check : EXIT 0
• pnpm exec biome check . : EXIT 0, 0 warnings
• pnpm test : tar-xz 190 → 203 passing (+13), 3 skipped unchanged ; full workspace 489 root + 203 tar-xz + 63 nxz = 755 passing, 0 fail

Test plan

• CI green
• Codecov reflects tar-xz at 100% lines
• Repo-wide coverage now at 100% lines (root + nxz + tar-xz)