feat: give the boards a horizon, a sea that reflects, and a state worth flying over

The wide shot, which is what an anonymous visitor actually lands on.

**The sea was `MeshLambertMaterial`** — a material with no specular term at all,
by construction — on a board where water is half the frame. It is now a
low-roughness dielectric that reads `scene.environment`, with a runtime-generated
tiling swell normal map sampled twice per fragment at two scales and two
headings, so the sun breaks into a moving glitter path instead of a mirror point.
An `onBeforeCompile` patch takes the body colour toward the deep value looking
straight down and leaves it to the reflection at grazing, and walks roughness up
past 1.6 board spans so the far water cannot shimmer.

The swell spectrum is 1/k^2 and not 1/k because the first attempt was
photographed: at 1/k every component carries the same slope, the shortest wave
wins, and the sea renders as hard diagonal corduroy. A test holds it now.

**The board no longer ends in a diamond.** The sea plane went from 1.8 board
spans to 18, past the fog's far plane from anywhere the orbit reaches, and the
sky is a world-space dome rather than a screen-space gradient. That gradient was
wrong in a way dusk made obvious: the sunset band was painted along the *bottom*
of the picture, under the board, while the true horizon at the top of frame stayed
zenith blue. `daylight.ts` pinning the horizon stop to the fog colour to hide the
seam was a symptom of it.

**Terrain casts shadows.** Left off before because double-sided terrain against a
~16 m-per-texel shadow map gives acne; `shadowSide = BackSide` is the cure, shot
at four sun elevations down to +0.0 degrees to confirm no stippling. The caster is
a stride-2 decimation appended to the same index buffer and swapped in by
`onBeforeShadow`/`onAfterShadow` via `drawRange`: no extra draw call, a quarter of
the depth cost, and indistinguishable from the full-resolution caster in a
side-by-side crop. Stride 1 was measured at +65,566 triangles and would have
missed the budget by ~47,000, so it was not shipped.

**California reads as California.** It was a beige kite: the eastern edge one
ruled line for five degrees of latitude, the south closing in a diagonal V, the
whole south-east a featureless tan wedge. Now the coast runs to the Mexican
border with San Diego on it, the eastern edge follows the Colorado and the Nevada
diagonal, and the south-east is the Basin and Range — forty parallel desert ridges
throwing shadows east, Death Valley as a white pan between the Panamints and the
Black Mountains, the Salton Sea the one cool value for two hundred kilometres.
The opening pose is retuned to the bigger board; the old 452/392 stand-off left a
slab of empty ocean where the state should be.

**The aircraft were six pixels.** Measured, by enlarging a screenshot 200% to
find one at all — indistinguishable from a dead pixel, on a board whose entire
claim is that the sky is live. They are airliners now, with planform and trail,
and clicking one raises its card for a signed-out visitor.

**The Model X is off the wall.** It stood at floor level outside a studio 188 m up
a Transbay tower, reading as a car balanced on a parapet. The apron is now chosen
from `site.elevation`, which the pack already carries — not from an office id,
which is the bug class this repo already hit once when a door marker gated on
`id === "sf"` and would have pinned the Los Angeles building to San Francisco.

Also fixed, and nearly shipped: sea z-fighting dithered every flat piece of ground
on the Bay Area and SoCal boards. And one test asserted an exact source line for
the water material, so the better multi-line implementation failed it — it now
asserts the property (dielectric, metalness 0, low roughness) rather than the
author's first guess at formatting.

Tests 964 -> 1015. California desktop 562/650 draw calls and 728,744/750,000
triangles — 2.8% of triangle headroom left, which is the number the next person
should check first. No budget was raised.

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

src/engine/types.ts

@@ -193,6 +193,23 @@ export interface ScenePalette {
   upland: number;
   park: number;
   parkHigh: number;
+  /**
+   * Bare high ground, and **optional on purpose**.
+   *
+   * The unpainted ramp is `flats → upland` over the first 150 m and then flat
+   * forever, which is right for a city — a hill in San Francisco is built to its
+   * summit and the buildings do the talking. On a board measured in hundreds of
+   * kilometres it is not: the Sierra crest at 4,000 m and the Mojave floor at
+   * 600 come out the same number, so a granite skyline and a creosote flat are
+   * one colour and the only thing separating them is the shading.
+   *
+   * A pack that declares this gets a second stop above `upland` — see
+   * `groundColor` in `terrain.ts` for the two elevations it ramps between. A
+   * pack that does not is rendered exactly as before, which is why this is
+   * optional rather than a tenth required colour that every existing pack would
+   * have to answer for.
+   */
+  alpine?: number;
 }
 
 // ---- Lighting -------------------------------------------------------------