Wayfinder map: R3F globe with zoom-driven terrain LOD

## Destination

A React + react-three-fiber app that draws the **whole Earth** as a WGS84 ellipsoid globe in Three.js, streaming Re:Earth Terrain quantized-mesh tiles (`https://terrain.reearth.land`), where **camera zoom drives terrain LOD** — the closer you get, the finer the terrain detail that streams in. Nothing else ships in this map: no physics, no UI panel, no other renderers.

The map is done when a browser opens the app, sees a full globe, zooms from space down to Mt Fuji (or any spot), and the terrain tessellation visibly refines with zoom — crack-free, on the ellipsoid, using this repo's own R3F terrain pipeline.

## Notes

- Domain: 3D globe rendering, quantized-mesh-1.0 terrain format, WGS84 ellipsoid math, quadtree LOD, three.js/react-three-fiber (R3F).
- Source of truth for the tiles being streamed: the **reearth-terrain** viewer at `/home/aditya/dev/reearth-terrain/viewer/index.html` and its `@navaramap/three` dependency — consult both before re-deciding anything.
- Standing decision (grilled & locked): **from-scratch R3F terrain engine**, not a wrapper around Navara. The render geometry must be ours to feed react-three-rapier later. Physics is a later map, out of scope here.
- Skills every session should consult: `codebase-design`, `domain-modeling`, `research`, `prototype`, `grilling`, `diagnosing-bugs`.
- Repo tracker: GitHub issues on `adityanandanx/nightcall` via `gh`.

## Decisions so far

- [Zoom-driven LOD: quadtree selection, crack-free](https://github.com/adityanandanx/nightcall/issues/6) — Working prototype `prototype/lod-quadtree` (commit 4dafb65): quad-locked **SSE quadtree** (Cesium level-zero error; refine while error>maxSSE) over TMS 4326 + **LRU TileCache** (byte budget + liveness stamps) + per-frame select→decode→render; **skirt** crack-mitigation (interim; production crack-free = child-from-parent edge upsampling per research #4); **RTC-center** geometry for float64 precision. Verified live: refinement monotone z2→z10 descending 15 Mm→40 km, 0 console errors. **Base branch for build.**
- [Prototype: one quantized-mesh tile rendered on the ellipsoid in R3F](https://github.com/adityanandanx/nightcall/issues/5) — Working prototype on branch `prototype/single-tile` (commit 649914b): Vite+React+R3F app fetches + decodes the Mt Fuji tile (z2/7/2), places it on the WGS84 ellipsoid via geodetic→ECEF, renders it. Verified in-browser: 4217 verts/8177 tris, correct radii, 0 console errors. Architecture confirmed workable; lazy-fetch + cache decoded meshes. Run: `npm i && npm run dev`.
- [Reference R3F/three.js globe-terrain LOD implementations to learn from](https://github.com/adityanandanx/nightcall/issues/4) — Crack-free = edge-matched child upsampled from parent mesh (+ skirts only for residual T-junctures). LOD = Cesium SSE formula + horizon-oct/occluder. Lifecycle = 3-tier queue + byte budget + liveness stamps. Steal from Cesium QuadtreePrimitive, Navara Rust core, 3DTilesRendererJS. Module split: decode→tile-cache→lod-selector→geometry-builder→drape→R3F meshes.
- [WGS84 ellipsoid math: placing geographic terrain tiles in a Three.js scene](https://github.com/adityanandanx/nightcall/issues/3) — Render in real meters; geodetic→ECEF (a=6378137, f=1/298.257223563; height on the geodetic normal). Tiling Geographic/TMS EPSG:4326 (z=0 is 2×1; y bottom-up). Globe datum = `ellipsoid` (ellipsoid=elevation+geoid; float64/RTC; max tile zoom from layer.json z0–14). NOTE: seam/skirt placement math deferred to #6.
- [Quantized-mesh-1.0 wire format: decoding terrain.reearth.land tiles](https://github.com/adityanandanx/nightcall/issues/2) — `.terrain` is a little-endian blob: 88-byte header, delta/zigzag u/v/h, high-water-mark indices, 4 edge lists, extension blocks (oct normals, watermask). Server: TMS/EPSG:4326 z0–14, 2 root tiles, extensions opt-in via `Accept` header only (URL params ignored), tiles uncompressed. Decode with a hand-rolled DataView reader; root/watermask tiles are big → lazy fetch + cache.

## Not yet specified

- Imagery/basemap draping over the terrain (EOX/OSM) — needed to make the globe legible, but deferred until terrain geometry works; graduates into a ticket after the decode→mesh→LOD frontier advances.
- Camera controls (orbit/zoom UX) — probably drei OrbitControls; exact interaction model deferred.
- Atmosphere / sky / ocean rendering at ellipsoid level — deferred.
- Scene scale & camera units (meters vs arbitrary) — deferred until the ellipsoid-placement ticket lands.

## Out of scope

- Physics (react-three-rapier) — later map.
- The full viewer UI panel (renderer/basemap switchers, location plaques) — later map.
- Cesium / MapLibre renderers — out of scope; this repo is R3F-only.
- PLATEAU buildings, demo orbit, iframe embed messaging, landing page.
- Deployment/hosting of the app.