Add durable IndexedDB storage for Code OSS browser profiles

[wieslawsoltes]

Code OSS falls back to in-memory user data when the native runtime has no IndexedDB implementation. This adds a host-configured durable storage subset with stable loopback profile identity, asynchronous atomic revisioned commits, quota and corruption errors, transaction rollback, cursor support, and the open/upgrade/versionchange flow used by the three VS Code web-state databases.

The change is reconstructed directly on current main at 649c2b63 (including the Web Crypto stack merged at 1c18c70c) and retains the focused Code OSS-compatible slice. It does not include the consolidation branch from #76.

The host supplies a private storage directory, stable application/profile partition key, and quota through Avalonia or Uno options. Storage remains absent unless the host opts in and the component manifest declares storage.indexeddb; an unavailable durable backend is never presented as a successful in-memory implementation.

Validation on macOS arm64 with pinned V8 15.3.10:

• webscene_indexeddb_storage_tests and webscene_native_indexeddb_contract: 2/2 passed against the linked current runtime.
• Candidate WPT-style profile: 1/1 document and 12/12 subtests passed against the newly built native library.
• Durability coverage includes rollback, quota, corruption, abandoned temporary writes, stale locks, cross-process stale-writer rejection, and restart across loopback port changes.
• Durable 4 KiB commit throughput: 1,670.73 commits/second; the regression gate requires 100 commits within ten seconds.
• SDK compatibility tests: 7/7 passed on .NET 10; Avalonia and Uno backend/SDK projects build with zero warnings and errors.
• Manifest and tooling contracts: 3/3 Node tests passed; the candidate manifest resolves the IndexedDB contract successfully.

This advances #56. That issue remains open for durable Blob/File MIME and prototype restoration; indexes, key paths, key generators, and IDBKeyRange; reverse and mutating cursors; complete transaction active-window semantics; broader cross-realm/process upgrade coordination; released-RID and Native AOT qualification; and packaged Frameforge media/project relaunch acceptance.

[wieslawsoltes]

Packaged Code OSS host verification is now green against the consolidated runtime containing this PR (bd139b26).

• real Code OSS workbench and native frames required before either storage stage
• first process committed a structured record with a Date
• host, WebScene engine, and bundled Node server were destroyed
• second process restored and type-checked the record on a different loopback port
• ports: 54790 → 54844
• duration: 8.758 seconds
• stable host partition: org.scenetech.code-oss-appscene:default
• quota: 512 MiB
• ordinary editor edit/undo smoke after storage integration: passed in 5.224 seconds

The host change is app-specific configuration only. It does not modify VS Code OSS or add a storage shim. Compact .wsidb evidence is retained while server/cache/profile artifacts are deleted after the gate.