[kitlangton]

Follow-up to #512.

What it does

After #512 the Barbara dock could still vanish or switch LOD at the wrong distance. Each forest chunk was culled and LOD-switched from its cell centre, and its box ignored the instanced trees and props that stick out of it. Now each chunk is culled by its own box. The far LOD also draws the dock and the camp string lights, so they don't disappear when the camera pulls back.

How it works

1. Each chunk keeps two boxes. bakedBox covers only the baked geometry. box is bakedBox grown to cover every instance's template extent.
2. box drives only the frustum test, so a tree leaning into view is never culled.
3. bakedBox drives the near/far switch, out-of-view retention and torch-caster reach. These use XZ distance to the box rather than to the cell centre, so the terrain near ring stays at 70 m. Instance LOD still uses cell distance.
4. The pure helpers (boxDistanceXZ, nearStatic, boxInFrustum) live in the new packages/kit/forest/cull.ts.
5. The far mesh adds a low-poly dock slab with posts and the string-light bulbs with posts, about 456 triangles.

Risk

The change is limited to the forest renderer in packages/kit/forest. Barbara is the only game that uses it. Growing the boxes keeps a few more chunks in view, which adds triangles but no measurable GPU time (table below). If a box were wrong, chunks would pop at the screen edge or the near/far switch would move.

At night, the dock still can't be seen from across the lake (70–100 m away), before or after this change. At that distance the deck is only a few pixels tall and nearly unlit. A test checks that the far mesh contains the deck, so it's covered even where a screenshot can't show it.

Proof

GPU frame time on the Mac mini at High quality, 5 alternating rounds, without → with instance extents (noise is about ±1 ms):

Shot	Median ms	Fastest ms	Triangles
beam	79.7 → 80.2	78.4 → 79.0	2.52M → 2.89M
k1	66.8 → 65.7	64.75 → 64.93	1.21M → 1.41M
dockFar	49.4 → 49.0	48.6 → 48.9	1.37M → 2.12M
camp	36.95 → 36.98	36.83 → 36.93	0.79M → 0.89M

campEast zoom, before (left) and after (right): before, only the glows showed; now the bulbs are drawn too.

• bun run verify: 49 steps pass
• kit forest tests 32/32, Barbara tests/forest-cull.test.ts 2/2 (the dock is kept in view from mid range, near/far uses box distance, the far mesh contains the deck)