fix: spec-store open-once read + fail-closed spec_id scan (iss-68)

[REPPL]

Resolves iss-68 (native ledger) — the last of the iss-65..72 sweep set.

Fixes (internal/core/spec)

• P7 — read TOCTOU. readRepoFile did Lstat then os.ReadFile, re-resolving the path (symlink-swap window). It now opens once with O_NOFOLLOW|O_NONBLOCK and validates the same fd (regular file, size cap) before reading. O_NONBLOCK keeps a FIFO/device leaf from blocking the open — a regression guard (TestLoadRejectsFifoSpecFile) enforces it, since the new open-first flow would otherwise hang where the old Lstat-first flow didn't.
• P5 — fail-open reservation scan. maxIntentSpecNum silently dropped a non-null intent spec_id from which no reservation number could be parsed, so its slot could be missed and NextID could mint a colliding id. It now fails closed on a spec_id with no parseable number (spc-, spc-abc). A spc-N or spc-N-<slug> form still reserves N — matching record-lint's planned prefix rule (^spc-) and the spec_lifecycle specNum trailing-slug tolerance, so the two gates agree and a lint-green record never bricks the mint path.
• seed4 — cosmetic. abcd intent plan seeds a spec with a clear author-guidance placeholder in ## Summary, not a bare TODO.

Document-accepted (trusted-worktree model, honest comments)

• seed5 — ensureDir guards the leaf but not a symlinked ancestor; planting one needs repo write access equal to editing the record directly.
• P8 — Close's Lstat→Rename clobber check is racy, but the atomic same-filesystem rename is preferred over a non-atomic no-clobber link+remove a crash could leave half-done; there is no concurrent adversary in-model.

Review

make preflight + make record-lint green; spec package green under -race. Reviewed over two rounds: ruthless SHIP; security BLOCK then PASS — the first cut used spec's anchored ^spc-[0-9]+$ (with a false comment that record-lint required it), which would have rejected a lint-valid spc-N-<slug> and hard-failed abcd intent plan repo-wide. Narrowed to specNumRe (has-a-number), corrected the comment, reverted the masking fixture, re-verified PASS.

Assisted-by: Claude:claude-opus-4-8