Add dashboard diagnostics metrics widget and settings toggle

[multiplex55]

Motivation

• Provide lightweight, throttled diagnostics for the dashboard to help developers observe FPS/frame time and per-widget refresh timings without adding runtime overhead.
• Surface cache sizes and slow widget refreshes with a dev-only UI so regressions or hotspots are easier to spot while developing.

Description

• Add a diagnostics metrics collector in src/dashboard/diagnostics.rs that records throttled frame timing and per-widget refresh durations and exposes DashboardDiagnosticsSnapshot and configuration constants like DIAGNOSTICS_REFRESH_INTERVAL and REFRESH_WARNING_THRESHOLD.
• Record per-widget refresh durations during dashboard rendering by instrumenting render_widget_content in src/dashboard/dashboard.rs and expose update_frame_timing/diagnostics_snapshot helpers on Dashboard.
• Add a diagnostics dashboard widget in src/dashboard/widgets/diagnostics.rs that displays FPS/frame time, cache sizes, per-widget last refresh time/duration and a small warning indicator for slow refreshes, and register it in WidgetRegistry (src/dashboard/widgets/mod.rs).
• Add a developer-only settings toggle show_dashboard_diagnostics to Settings (src/settings.rs), expose it in the settings editor UI (src/settings_editor.rs), and wire it into the app (src/gui/mod.rs) so the diagnostics snapshot is passed into DashboardContext when enabled.
• Include unit tests for diagnostics behavior in src/dashboard/diagnostics.rs that verify frame metric throttling and threshold-triggered widget refresh updates.

Testing

• Ran the workspace tests focused on diagnostics with cargo test diagnostics; compilation started but the run failed due to a missing system library dependency: alsa required by alsa-sys (pkg-config could not find alsa.pc), so tests did not complete.
• Unit tests were added for DashboardDiagnostics (throttling and warning threshold behavior) and are expected to pass when system build dependencies are available and cargo test completes.

---

Codex Task