WebGL context-loss recovery practice

Roadmap item 2.1.2 introduces context-loss fallback and retry behaviour. Roadmap item 2.2.1 adds WebGL allocation and fallback metrics. Follow these rules when changing context-loss handling and renderer instrumentation:

• Keep context-loss handling in lib/components/term.tsx wired through onWebGLContextLoss.
• On context-loss or pool-eviction events, immediately detach the WebGL addon and attach Canvas.
• Keep retry behaviour bounded via existing failure controls: webglFailureCount, webglLastFailureAt, webglCooldownMs, webglFailureThreshold, and webglFailureDecayMs.
• Keep deterministic retry scheduling in scheduleDeterministicRendererRetry() and preserve timer cleanup in componentWillUnmount() to avoid leaked retries.
• Keep retry entry through visibility/pool coordination (syncRendererForVisibility and getWebGLContextPool(...)) so retries occur only when pane visibility and pool capacity allow a safe WebGL reattach.
• Keep fallback instrumentation observable through: console.warn('WebGL context lost. Falling back to canvas-based rendering.') and renderer mode events from Term.reportRenderer(...) (info renderer, with optional reason values such as context-loss, pool-evicted, and webgl-init-failed), which feed renderer summaries in the About dialog.
• Keep allocation metrics aligned with renderer telemetry from Term.reportRenderer(...) and main-process aggregation in app/utils/renderer-utils.ts:
• Current WebGL context count (current) tracks active WebGL renderer assignments.
• Peak WebGL context count (peak) tracks the highest observed current value during the process lifetime.
• Keep diagnostics visibility in the About dialog detail output by including:
• WebGL contexts: current <n>, peak <n>
• Renderer fallbacks: total <n>; reasons: ...
• When tuning context-loss retry thresholds or fallback behaviour, update docs/roadmap.md, this guide, and the related renderer/pool unit tests in the same change to avoid docs/runtime drift.
• When adding or renaming fallback reasons or allocation metric fields, update this guide, docs/roadmap.md, and diagnostics tests in the same change.