feat(preview): one full California, behind ?nest=1

Not finished, off by default, and shipped anyway — for the reason `reconcile`
was shipped behind a flag: the owner is choosing between two shapes of the
product and an argument about how a board looks is settled by looking at it.

`?nest=1` draws San Francisco's and Southern California's ground inside the
state board instead of beside it. The question it answers is whether one full
California needs a terrain quadtree, or whether the three packs already are the
detail tiers of one map and only ever needed drawing in one scene.

**They are, and the placement is a similarity transform and nothing more.**
`World.project` is a uniform scale in x/z with no vertical term, so a metro
enters the state board's space by scaling x/z by the ratio of metres-per-unit,
scaling y by the ratio of `exaggeration / metresPerUnit` — separately, because
the two boards stretch height differently — and translating to where the state
board projects the metro's own centre. Measured: sf is xz 0.04915, y 0.12764 at
(-146.2, -27.5) on a 551-unit board; socal is xz 0.20351, y 0.89640 at
(55.4, 201.8). Both land where they belong at the size they should be, and the
close pose shows real towers, blocks and a freeway on the state board.

**And the statewide lattice is not the blocker TODO.md says it is.** That file
records a statewide lattice at metro fidelity as 23.7M points and dead. That is
true of `buildAxis`, which refines per *axis* — a focus region refines its whole
row and its whole column, a plus rather than a box — so two metros at opposite
corners refine most of the state. Measured against the packs' real focus regions
(sf's is 22x40 km; socal's two are 9x12 and 8x13), per-*rectangle* refinement is
**2.93M points and about 26 MB** at a 720 m coarse cell, or 1.13M at 1439 m,
against 0.75M for all three boards as they stand today. The verdict in TODO is a
fact about the lattice's shape, not about one California.

**What is broken in the preview, precisely.** Two terrains occupy the same
ground: the state's coarse mounds are still drawn under the metro's fine surface
and they interpenetrate, so a block sits on a smooth hill in one place and is
buried in it three hundred metres away. That is nested-grid stitching — cut a
hole in the coarse lattice over each fine rectangle and sew the seam — and it is
the whole of the remaining work. Nothing about the transform is implicated. Also
not done: no LOD decides which ground draws, the metros are lit by a rig that
belongs to another board, and triangles are additive rather than traded.

`water` and `sea` are deliberately not lifted. The state board already has an
ocean at y=0 and a second one inside it is a z-fight rather than a sea — the same
defect the terrain still has, and the one place it was cheap to avoid.

Inert without the flag, verified: `?city=california` with and without `?nest=1`
both boot clean with zero page errors, and 1,694 tests pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/main.ts

@@ -151,7 +151,7 @@ import {
   type WebcamFaceTextureAdapter,
 } from "./profile/index.ts";
 import type { ActorIdentity } from "./actors/controller.ts";
-import { SRGBColorSpace, Vector3, VideoTexture } from "three";
+import { Group, SRGBColorSpace, Vector3, VideoTexture } from "three";
 import {
   CALIFORNIA_AIR_ROUTE,
   createAircraftPoseSnapshot,
@@ -2112,6 +2112,92 @@ async function buildBoard(
  * `prefers-reduced-motion` collapses the whole thing to a cut, exactly as
  * `arrive()` and `beginOfficeArrival` already do.
  */
+/**
+ * One full California, as a **preview behind `?nest=1`. Off by default, and it
+ * is not finished.**
+ *
+ * ## What it is for
+ *
+ * The owner has asked for one map of the whole state rather than three boards,
+ * and the question that had to be answered before anyone spent a week on it was
+ * whether the three packs already *are* the detail tiers of one map and only
+ * ever needed drawing in one scene. They are. This is the evidence, shipped
+ * behind a flag for the same reason `reconcile` was: an argument about how a
+ * board looks is settled by looking at it.
+ *
+ * ## Why the placement is a similarity and nothing more
+ *
+ * `World.project` is a uniform scale in x/z with no vertical term, so a metro's
+ * ground enters the state board's space under a plain similarity transform:
+ * scale x/z by the ratio of metres-per-unit, scale y by the ratio of
+ * `exaggeration / metresPerUnit` — separately, because the two boards stretch
+ * height by different amounts — and translate to where the state board projects
+ * the metro's own centre. Measured: San Francisco is xz 0.04915, y 0.12764, at
+ * (-146.2, -27.5) on a 551-unit board; Southern California is xz 0.20351,
+ * y 0.89640, at (55.4, 201.8). Both land where they should, at the size they
+ * should be.
+ *
+ * ## What is broken in it, precisely
+ *
+ * **Two terrains occupy the same ground.** The state board's coarse mounds are
+ * still drawn under the metro's fine surface, so they interpenetrate: a block
+ * sits on a smooth coarse hill in one place and is buried in it three hundred
+ * metres away. That is the nested-grid stitching problem and it is the whole of
+ * the remaining work — the coarse lattice needs a hole cut over each fine
+ * rectangle and the seam sewn. Nothing about the transform above is implicated.
+ *
+ * Also not done, and visible: the metros keep their own lighting-free lift into
+ * a scene whose rig belongs to the state board, no LOD decides which of the two
+ * grounds should draw, and the triangles are additive rather than traded.
+ *
+ * ## Why the cheap statewide lattice is possible at all
+ *
+ * `buildAxis` refines per **axis** — a focus region refines its whole row and
+ * its whole column, a plus rather than a box — which is why `TODO.md` records a
+ * statewide lattice at metro fidelity as 23.7M points and dead. Measured against
+ * the packs' real focus regions, per-*rectangle* refinement is **2.93M points
+ * and about 26 MB** at a 720 m coarse cell, against 0.75M for all three boards
+ * as they stand. The verdict in TODO is a fact about the lattice's shape, not
+ * about one California.
+ */
+const NEST = new URLSearchParams(location.search).get("nest") === "1";
+
+/** The ground of a board, without its sky, its sea or its rig. */
+const NESTED_LAYERS = ["terrain", "shorePlates", "blocks", "landmarks"] as const;
+
+async function nestMetros(host: MountedBoard): Promise<void> {
+  if (!NEST || host.id !== "california") return;
+  const hostWorld = host.handle.world;
+  for (const id of ["sf", "socal"]) {
+    const entry = CITIES.find((candidate) => candidate.id === id);
+    if (entry === undefined) continue;
+    const built = await buildBoard(entry, new AbortController(), { quiet: true });
+    if (built === null) continue;
+    const metro = built.handle.world;
+    const group = new Group();
+    group.name = `nested:${id}`;
+    const xz = metro.metresPerUnit / hostWorld.metresPerUnit;
+    const y =
+      (hostWorld.city.verticalExaggeration / hostWorld.metresPerUnit) /
+      (metro.city.verticalExaggeration / metro.metresPerUnit);
+    const [px, pz] = hostWorld.project(metro.city.center.lat, metro.city.center.lng);
+    group.scale.set(xz, y, xz);
+    group.position.set(px, 0, pz);
+    /*
+     * `water` and `sea` are deliberately left behind. The state board already
+     * has an ocean at y=0 and a second one nested inside it is a z-fight rather
+     * than a sea — which is the same defect the terrain still has, and the one
+     * place it was cheap to avoid.
+     */
+    for (const name of NESTED_LAYERS) {
+      const source = built.handle.stageScene.scene.getObjectByName(name);
+      if (source === undefined) continue;
+      group.add(source);
+    }
+    host.handle.stageScene.scene.add(group);
+  }
+}
+
 async function presentBoard(record: MountedBoard): Promise<void> {
   const outgoing = visibleBoard;
   if (outgoing === record) {
@@ -2152,6 +2238,8 @@ async function presentBoard(record: MountedBoard): Promise<void> {
 
   activateBoard(record);
   hideSwitchProgress();
+  // The one-California preview. A no-op unless `?nest=1`; see `nestMetros`.
+  void nestMetros(record);
   /*
    * Arriving is itself a reason to look ahead.
    *