[POSIX] Keep directory streams usable when rewind fails

[brandonpayton]

Why

Kandelo sometimes recreates a host directory iterator to move a directory stream back to the beginning. The legacy rewinddir path closed the live iterator before opening its replacement. If opening the replacement failed, the kernel still pointed at a handle that was already closed, so the caller could no longer continue reading from its previous position.

The directory-entry retry cache added in #1060 is cleaned up in every reachable production lifecycle path, but its failure edges were not directly tested. A later change could accidentally retain an entry after a failed close or replay an entry that belongs to an older use of the same numeric handle.

What changed

• Open and validate a replacement directory iterator before retiring the live one.
• Preserve the original iterator and position if opening fails.
• If closing the original iterator fails, discard the unused replacement and keep the original stream authoritative.
• Document that a PlatformIO directory handle is opaque and cannot identify two live iterators at once.
• Add direct bridge tests for failed close cleanup, defensive numeric-handle reuse cleanup, and a name-buffer failure after metadata was already written.
• Document the resulting rewinddir behavior in the POSIX status reference.

Platform contracts

This protects POSIX directory-stream state and the shared host bridge used by both Node.js and browsers. It does not change the Wasm ABI, package artifacts, or VFS image format.

A WeakMap is intentionally not used. Directory handles cross the Wasm boundary as numbers, while WeakMap requires object keys and garbage collection cannot own POSIX resource lifetime. A future ownership consolidation, if needed, should be a strongly owned directory-record registry or the planned machine-wide open-file-description table.

Validation

• ./scripts/dev-shell.sh bash scripts/ci-run-test-suite.sh cargo-kernel — 1,246 passed.
• Focused host/test/readdir-atomicity.test.ts Vitest run without unrelated guest-fixture setup — 5 passed.
• ./scripts/dev-shell.sh bash -lc "cd host && npm run typecheck" — passed.
• git diff --check — passed.

The standard full Vitest setup was not used as local evidence because this clean worktree has no current musl sysroot. Repository CI will build matching guest fixtures and run the complete host suite.

[github-actions]

Phase B-1 matrix build status — pr-1063-staging

ABI v41. 6 built, 0 failed, 6 total.

Package	Arch	Status	Sha
kandelo-sdk	wasm32	built	e2df6499
rootfs	wasm32	built	a0d29e15
shell	wasm32	built	7a287cb2
lamp	wasm32	built	f7c8d643
node-vfs	wasm32	built	92c72b70
wordpress	wasm32	built	2373060b

_{Auto-generated; replaced on each push. Raw data in the publish-status workflow artifact.}