Add layered memory capture APIs and demos

[timazed]

Summary

• add a layered memory API with low-level raw records, mid-level MemoryWriter / captureMemories(...), and high-level opt-in automatic capture policies
• fix memory ranking and budget-selection issues in the query engine
• add runtime memory capture hooks and observer events
• add a dedicated Memory demo tab showing high-, mid-, and low-level flows
• update README and test coverage for the new memory model

Verification

• swift test
• xcodebuild -project DemoApp/AssistantRuntimeDemoApp.xcodeproj -scheme AssistantRuntimeDemoApp -destination 'generic/platform=iOS Simulator' CODE_SIGNING_ALLOWED=NO build

Notes

This branch targets main, which is the current 2.0 development trunk.