3D replay: auto theme default, speed + glide-ratio trail colours, camera continuity across backdrop switches

[pokle]

Follow-up to #230.

Auto is the default theme

With nothing stored, the replay now follows the OS colour scheme ('system') instead of defaulting to dark — and keeps following it live. Users who explicitly picked Dark or Light are unaffected (their stored choice wins).

New trail colouring: Speed

Colours the trail by ground speed — useful for reading strategy: thermalling drift vs cruise vs full-bar glides jump out at a glance.

• Per-vertex smoothed ground speed is computed at load in assembleTracks (horizontal path length over the same ±3-fix window as the vario smoothing, via a per-pilot prefix sum — no inner loop), fed to the trail shader as a new aSpeed attribute shared by the line and points passes.
• Rendered on the sequential ramp from SPEED_MIN = 18 km/h to SPEED_MAX = 108 km/h. The floor isn't 0 — nobody flies below stall speed, and anchoring the ramp at 0 wasted its bottom third and compressed the useful range (first cut did exactly that; re-ranged after eyeballing).

New trail colouring: Glide ratio

• Red (poor) → yellow → green (good), scaled logarithmically over ratios 2…32 — ratios span a large range and a 4→8 improvement should read as strongly as 16→32 (t = log2(g/2)/log2(16) in the shader, from aSpeed / −aVario).
• Climbing/level segments render flat in the themed vario-zero colour — glide is effectively infinite there, and the flat grey de-emphasises climbs so the glide quality between thermals carries the picture.

Both modes get colour-scale legends: speed in the user's preferred units (≤18 km/h … ≥108 km/h), glide as ratios with the geometric-mean midpoint (≤2 · 8 · ≥32 · climb = grey).

Camera continuity across backdrop (and theme) switches

Switching Abstract ↔ Map used to snap to each backend's default framing — a jarring loss of continuity. Both camera models reduce to the same five numbers, so rebuild() now captures a backend-agnostic ViewState from the outgoing backend and seeds the incoming one (setInitialView before mount(), applied instead of the default whole-task framing):

• look-at point in local ENU (y = 0 from the map side — it has no elevated look-at),
• bearingDeg (shared compass convention), pitchDeg (0 = straight down; abstract's polar angle maps 1:1, clamped to Mapbox's 85° max),
• mpp — metres per pixel at the view centre, which carries the zoom without either side knowing the other's projection model: abstract camera distance = mpp·viewportH / (2·tan(fov/2)); map zoom = log2(40075016.686·cos(lat)/mpp) − 9, the exact inverse of its getMetresPerPixel.

Theme switches ride the same path, so they no longer reset the camera either.

Verification

• bun run typecheck:all passes; replay unit tests 17/17 (terrain-follow, new terrain-view round-trip: applyView → getViewState recovers x/z/bearing/pitch/mpp exactly, pitch clamps at 85°, zoom accounts for latitude).
• Playwright: auto default (light OS → light page, nothing stored; follows a live OS flip); Speed and Glide modes render with correct legends and no shader errors; camera continuity — after orbiting to bearing 63° and zooming, a backend rebuild preserves the compass bearing within 0.5° and the scale bar to the pixel; the full regression suite still passes.
• Docs updated (docs/3d-flight-replay-notes.md §5.14b).

🤖 Generated with Claude Code

https://claude.ai/code/session_01DZpFYsCEyR7ck3bWmQNhCr

[github-actions]

Preview Deployment
https://ea70bd4f.glidecomp.pages.dev
Commit: 6ba3fce