feat(symbian): package private mass-storage data

[doodlewind]

What changed

• add --mass-storage-data-root <dir> for application-specific Symbian cores
• stage a deterministic regular-file tree inside the serialized build transaction
• reject symlinks, unsafe paths, case-insensitive collisions, special files, empty trees, and canonical source/payload overlap
• revalidate the exact file set, byte counts, and SHA-256 digests in the offline container
• install data to E:\private\<UID>\data\<relative> and record {path, bytes, sha256} in receipt schema 3
• document the custom-core boundary and include the new hermetic tests in the governed test command

Why

Large native assets must not be embedded in app.pak: the Symbian host and QuickJS can otherwise duplicate the full pack in the E7's small process heap. This gives custom native cores a generic, receipt-pinned data channel while keeping the stock runtime and launcher contracts unchanged.

Impact

Existing apps continue to receive an empty data array. Only callers that explicitly combine --core-library with --mass-storage-data-root install external files, and the native core remains responsible for opening them.

Validation

• bun node_modules/typescript/bin/tsc --noEmit
• bash -n tools/symbian/container/pocketjs-symbian-build-app
• bun test tests/symbian-data.test.ts — 3/3
• bun test tests/symbian-runtime.test.ts — 13/13
• bun test tests/symbian-toolchain.test.ts --test-name-pattern '^(?!.*CODA launch wire)' — 13/13
• bun test tests/npm-package.test.ts — 3/3
• git diff --check

The excluded existing CODA launch fixture is locally blocked before test logic because endpoint security stalls its compiled executable in dyld; the mass-storage patch does not touch CODA.