browser-vfs: require shell for SQLite test images

[brandonpayton]

Summary:

• Add a reusable installPosixShell VFS helper that installs dash plus /bin/sh, /usr/bin/dash, and /usr/bin/sh.
• Make the SQLite browser test VFS image require dash instead of silently omitting sh.
• Add focused helper coverage and document the shell requirement for VFS images that run shell-mediated commands.

Validation:

• Reproduced kd-d7q rerun4 shell failure from logs: crash3 and walcrash3 rows attempted /usr/bin/sh and reported cannot execute sh / no such file or directory.
• Focused host test: cd host && ../scripts/dev-shell.sh npx vitest run test/vfs-image-helpers.test.ts passed, 1 file / 1 test.
• ABI: scripts/dev-shell.sh bash scripts/check-abi-version.sh passed; snapshot, libc header, and TS bindings in sync.
• SQLite VFS build: scripts/dev-shell.sh env SQLITE_TEST_VFS_OUT=test-runs/kd-i95-verify-current/sqlite-test.vfs.zst bash images/vfs/scripts/build-sqlite-test-vfs-image.sh passed.
• VFS inspection: /bin/dash mode 755 size 492175; /bin/sh -> /bin/dash; /usr/bin/dash -> /bin/dash; /usr/bin/sh -> /bin/dash.
• Browser smoke: scripts/dev-shell.sh ./run.sh browser --host 127.0.0.1 --port 6504 reached Vite ready; /pages/sqlite-test/ returned 200; /sqlite-test.vfs.zst returned 200 with Content-Length 12394062 and COOP/COEP headers.
• Kernel lib: scripts/dev-shell.sh cargo test -p kandelo --target aarch64-apple-darwin --lib passed, 959 passed / 0 failed.
• POSIX: scripts/dev-shell.sh bash scripts/run-posix-tests.sh exited 0, 174 pass / 0 fail / 3 xfail / 2 skip / 0 timeout.
• libc: scripts/dev-shell.sh bash scripts/run-libc-tests.sh exited 0, 302 pass / 0 fail / 20 xfail / 1 flaky pass / 1 timeout regression/raise-race / total 324.
• Full host Vitest: cd host && ../scripts/dev-shell.sh npx vitest run failed with 1 unrelated artifact failure in packages/registry/spidermonkey/test/spidermonkey-node-compat.test.ts: npm package installation cannot find /usr/local/lib/kandelo/npm-runner.js. Summary: 110 files passed / 7 skipped / 1 failed; 839 tests passed / 21 skipped / 1 failed.

Node/browser parity:

• This change is in shared VFS image construction, not host-specific runtime code.
• Node and browser both restore MemoryFileSystem images through shared image helpers; browser-specific verification used the SQLite VFS asset through ./run.sh browser.

[github-actions]

Phase B-1 matrix build status — pr-763-staging

ABI v15. 5 built, 0 failed, 5 total.

Package	Arch	Status	Sha
kandelo-sdk	wasm32	built	16e5ce9c
shell	wasm32	built	2985ab00
lamp	wasm32	built	34cd23d7
node-vfs	wasm32	built	affe64bf
wordpress	wasm32	built	e34493c6

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