vfs: preserve open unlinked SharedFS files

[brandonpayton]

Summary

Fixes the SharedFS file-lifetime bug behind the SQLite sort2 temp-file failure: SQLite opens /tmp/etilqs_*, unlinks it, and continues positioned I/O through the open fd. SharedFS previously removed and freed the inode at unlink time, so a later temp file could reuse/reset that inode while the original fd still existed. A later pread on the old fd could then see EOF or replacement contents instead of the bytes previously written through that fd.

This PR keeps unlinked or rename-replaced inodes alive until the last fd closes, adds direct readAt/writeAt operations to SharedFS, and routes MemoryFileSystem pread/pwrite through those operations instead of mutating the shared fd offset.

Validation

Passed:

• bash scripts/dev-shell.sh bash -lc 'cd host && npx vitest run test/vfs/sharedfs-positioned-io.test.ts'\n - 1 file passed, 4 tests passed\n- bash scripts/dev-shell.sh bash scripts/check-abi-version.sh\n - ABI snapshot/header/TS bindings up to date\n\nRun but red from unrelated/baseline areas on this clean SharedFS-only branch:\n\n- bash scripts/dev-shell.sh bash -lc 'cd host && npx vitest run'\n - 91 files passed, 25 skipped, 5 failed\n - 755 tests passed, 111 skipped, 2 expected fail, 9 failed\n - failing areas: test/fork-from-thread.test.ts, test/fork-instrument-coverage.test.ts pthread/fork cases, test/pthread-trap-semantics.test.ts, test/pthread.test.ts, and test/wasm64.test.ts missing programs/wasm64/hello64.wasm\n - the new test/vfs/sharedfs-positioned-io.test.ts passed in this full run\n\nPreviously recorded on bead kd-c5i.11 before creating this clean PR branch:\n\n- official browser ^memsubsys1 sort2.test$ passed 1/1 host job, 11 SQLite cases, 0 errors on the validated stacked worktree\n- official Node ^memsubsys1 sort2.test$ passed 1/1 host job, 11 SQLite cases, 0 errors on the validated stacked worktree\n\n## Notes\n\nThis draft PR intentionally contains only the SharedFS root-cause fix, not the earlier stacked SQLite/pthread work from origin/gascity/kd-cfo/kd-c5i.11-diagnose-official-memsubsys1-sort2-browser-timeout.\n\nNo ABI change is expected or observed. No user-facing docs update is included because this is a VFS correctness repair for existing POSIX open-unlink semantics, covered by focused runtime tests.

[github-actions]

Phase B-1 matrix build status — pr-786-staging

ABI v16. 6 built, 0 failed, 6 total.

Package	Arch	Status	Sha
kandelo-sdk	wasm32	built	d5b38af6
rootfs	wasm32	built	a4db6ad3
shell	wasm32	built	95517855
lamp	wasm32	built	bfeb3682
node-vfs	wasm32	built	b38d0c40
wordpress	wasm32	built	0da1d75e

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