Add Astra editor and retained compositor

[ExtraBinoss]

Summary

• add the polished native/Web Astra code-editor example with responsive explorer, tabs, Rust highlighting, project search, split panes, native terminal launch, and fast file editing
• add incremental TextEdit delivery, retained compositor layers, composition-only caret updates, deadline-based frame scheduling, and coalesced runtime events
• document the interaction/editing/compositor APIs and add native performance plus browser verification tooling

Verification

• ./scripts/quality.sh
• 1399 tests passed, 2 skipped
• workspace coverage: 88.24% branches, 93.75% functions, 94.55% lines, 93.65% regions
• native performance profile passed every budget (0.67% idle CPU, ~6759 typing events/s)
• wasm-pack Web build and hidden Chromium responsive smoke test passed

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