[feat](trx-frontend-http): migrate frontend canvas rendering to WebGL

Replace Canvas2D rendering in spectrum, overview, signal overlay, and CW tone picker with a shared WebGL renderer and wire the new asset into frontend HTTP routes.\n\nCo-authored-by: OpenAI Codex <codex@openai.com>

Signed-off-by: Stan Grams <sjg@haxx.space>

aidocs/WEBGL.md

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+# Canvas2D to WebGL Transition Plan
+
+## Goal
+- Replace all runtime Canvas2D rendering in the frontend with WebGL.
+- Remove Canvas2D code paths after feature parity is reached.
+- Keep existing interaction behavior (zoom/pan/tune/BW drag/tooltips/overlays) intact.
+
+## Scope
+- `src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js`
+  - `overview-canvas`
+  - `spectrum-canvas`
+  - `signal-overlay-canvas`
+- `src/trx-client/trx-frontend/trx-frontend-http/assets/web/plugins/cw.js`
+  - `cw-tone-waterfall`
+- New shared WebGL utility module:
+  - `assets/web/webgl-renderer.js`
+
+## Non-Goals
+- No Canvas2D fallback path.
+- No feature redesign outside rendering internals.
+
+## Constraints
+- Must preserve existing data flow and event wiring.
+- Must keep map/decoder/bookmark integrations unchanged.
+- Must remain dependency-free (no external rendering libraries).
+
+## 2-Phase Migration
+1. Phase 1 (Rendering engine insertion)
+- Add shared WebGL renderer utility (primitives + textures + color parsing).
+- Keep existing business logic and interaction handlers untouched.
+- Swap draw targets from 2D contexts to WebGL primitives.
+
+2. Phase 2 (Canvas2D removal and parity closure)
+- Remove `getContext("2d")` usage from app and plugins.
+- Remove obsolete 2D-specific cache paths.
+- Validate behavior on resize/theme/style/stream reconnect/decoder mode changes.
+
+## Parallel Workstreams ("Agents")
+1. Agent A: Shared WebGL core
+- Build `webgl-renderer.js` with:
+  - HiDPI resize handling
+  - Solid/gradient rects
+  - Polyline/segment/fill primitives
+  - RGBA texture upload + blit
+  - CSS color parser helpers
+
+2. Agent B: Main spectrum/overview migration
+- Port `drawSpectrum`, `drawHeaderSignalGraph`, `drawSignalOverlay`, and clear paths.
+- Replace 2D offscreen waterfall cache with WebGL texture updates.
+- Keep frequency axis/bookmark axis DOM behavior unchanged.
+
+3. Agent C: CW tone picker migration
+- Port `drawCwTonePicker` primitives to WebGL.
+- Preserve auto/manual tone interactions and mode gating.
+
+## Acceptance Criteria
+- No frontend `getContext("2d")` usage remains.
+- All four canvases render using WebGL and respond to resize/DPR changes.
+- Spectrum interactions still work:
+  - wheel zoom
+  - drag pan
+  - BW edge drag
+  - click tune
+- Overview strip continues showing waterfall/history.
+- CW tone picker remains interactive and reflects current spectrum/tone.
+
+## Verification Checklist
+- Static:
+  - `rg -n 'getContext\\("2d"\\)' src/trx-client/trx-frontend/trx-frontend-http/assets/web`
+- Runtime smoke:
+  - Open main tab: verify overview + spectrum + overlay.
+  - Toggle theme/style.
+  - Resize window and spectrum grip.
+  - Enable CW decoder and validate tone picker updates/click-to-set.
+  - Confirm no rendering exceptions in browser console.
+
+## Rollout Notes
+- Initial rollout is WebGL-only.
+- If a browser lacks WebGL, canvases remain blank by design until a dedicated fallback policy is defined.