Add Web and in-memory backends

[timcreatedit]

Summary

• add a shared pure-Dart engine and synchronous diskless IsarMemory helper
• automatically use IndexedDB on web with per-collection records, schema migration checks, revision safety, and single-tab ownership
• replace the legacy TypeScript/CDN bridge with modern JS interop and Dart Wasm support
• add backend capability handling, full memory conformance coverage, browser tests, and JS/Wasm CI gates

Verification

• full memory backend conformance suite passes
• focused memory engine tests pass
• IndexedDB browser tests pass under JavaScript and Dart Wasm
• generator tests pass
• JavaScript and Wasm compilation gates pass
• inspector builds successfully for JavaScript and Wasm

Limitations

• web remains async-only and single-tab writer ownership is enforced
• filesystem, isolate, configured-size, and disk-migration tests are intentionally capability-skipped for the disposable memory backend
• native core verification was not run locally because Cargo is unavailable