fix(tar-extract): judge the entry name that is written; bound pax work per archive

[PrzemekGalarowicz]

What this changes

Fix 4 of the post-review batch (review of PHARN-01..18), shipped through /pharn-dev-ship.

• Name check matches the written name. ustar name/prefix are now decoded as strict UTF-8 (TextDecoder fatal: true, ignoreBOM: true). Before, a lenient decode turned invalid bytes (e.g. raw C1 0x85) into U+FFFD, so hasUnsafeChars judged a different string from the one written. Invalid UTF-8 or a leading BOM now refuses the archive.
• Pax work bounded per archive. Global (g) pax headers total at most 64 KiB per archive, and each one counts toward maxEntries. Before, the limits applied per header, so the total was unbounded.
• No raw header bytes in messages. The non-octal numeric-field and bad-UTF-8 errors render bytes through describeBytes, which escapes them as \xNN and caps the length.

Type of change

• feat — new stack option, wizard step, or command capability
• fix — bug fix
• docs — docs-only change
• chore / refactor — tooling or internal restructure, no behavior change

Area(s) touched

src/lib/tar-extract.ts (fetch boundary), tests/tar-extract.test.ts, CHANGELOG.md (### Security), .dev/features/tar-entry-names/ (pipeline artifacts).

Checklist

• Read the existing file(s) before editing; followed the ESM .js-extension import convention.
• Updated the matching tests/*.test.ts — 9 new cases; 8 fail on the base code, and the BOM case is a guard.
• Updated the relevant docs/ page — N/A (no user-facing surface beyond CHANGELOG).
• Preserved the security invariants — this change only narrows what an untrusted archive may contain.

Quality gates

• npm run check passes locally (format:check + lint + typecheck + test) — 1507 tests.
• npm run build succeeds.
• npm run test:coverage passes (coverage thresholds met).

Notes for the reviewer

• Pipeline results: validate exit 0, regress no-regressions, verify PASS, review GREEN with 2 minor advisory findings. See .dev/features/tar-entry-names/REVIEW.md.
• A real git-archive of pharn-oss still extracts to the same 2208 files / 414 dirs.
• Advisory: a single non-UTF-8 filename upstream would now fail every install. pharn-oss uses only ASCII names today.

🤖 Generated with Claude Code

https://claude.ai/code/session_0199owRmYfskqYQVQrVP679o

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Run ID: 9cb942bb-0f0a-4530-a230-ef0a39a53fb2

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