Date: 2026-03-01
Issue: #180
Domain: Runtime-viewer bridge precondition for egui_glow -> egui_wgpu

Land additive diagnostics instrumentation that captures bridge-readiness metrics without changing the active renderer backend.

1. Added new diagnostics channels:
   • diagnostics.compositor_bridge_probe
   • diagnostics.compositor_bridge_probe.failed_frame
   • diagnostics.compositor_bridge_probe.callback_us_sample
   • diagnostics.compositor_bridge_probe.presentation_us_sample
2. Emitted bridge probe and sample events from guarded compositor callback path in shell/desktop/workbench/compositor_adapter.rs.
3. Extended diagnostics replay summary and compositor diagnostics pane with:
   • bridge_probe_count
   • bridge_failed_frame_count
   • avg_bridge_callback_us
   • avg_bridge_presentation_us
4. Registered channels in diagnostics registry with explicit severities.
5. Added focused tests for bridge probe channel aggregation and snapshot summary values.

• cargo check -q
• cargo test -q --lib replay_channels_emit_for_sample_and_violation_artifact
• cargo test -q --lib snapshot_json_includes_compositor_bridge_probe_summary_metrics

• This slice is backend-neutral and does not replace egui_glow.
• #183 remains blocked pending completion of #180 bridge viability evidence collection.