Collision BVH is built on every twin route — ~780ms of work orbit/ortho/tour cannot use

[TortoiseWolfe]

The full collision BVH is built on every twin route, including ones that can never use
it. Measured at ~780 ms of synchronous main-thread work, and it is the single largest
identified contributor to the freeze in #717.

Evidence

CDP CPU profile of /twins/chatt/?ortho, self time:

_buildNodes           364 ms + 192 ms   binned-SAH node builder (src/lib/cod/bvh.js)
_nodeBoundsFromRange   97 ms +  50 ms
StaticWorld.build      74 ms
                      ----------------
                      ~780 ms

Why it happens

tryBuildWalk() in src/twin/TwinCanvas.client.tsx gates only on:

if (!buildings || !terrain || !groundAt) return;

There is no mode check — the "mode" occurrences inside the function are all in comments.
It is called from handleBuildingsMesh, handleTerrainMesh and handleGroundReady, each
of which fires as soon as its data lands, on every route the twin renders.

Nothing outside walk consumes it

• ?ortho maps to rig mode orbit: const rigMode: RigMode = mode === 'ortho' ? 'orbit' : mode
• Rig.collide is invoked only inside _walk() (src/stage/Rig.ts:573); _tour, _orbit
  and _follow never call it
• rig.walkMove is the embodied-walk seam and is only driven in walk

So orbit, ortho, tour and follow each pay ~780 ms for a structure they cannot touch.

Fix

Defer the build until walk mode is entered. The machinery already exists and was built for
earlier fixes in this arc:

• tryBuildWalkRef is already the deferred-call seam (added for [diorama-walk] Walk mode (/chatt?diorama&walk) spawns you below the city — you fall forever #651's ordering race), so
  triggering later is wiring rather than new machinery.
• The [diorama-walk] the parked bike teleports back to spawn because the walk controller silently rebuilds #703 idempotence guard (builtFromRef comparing the terrain / buildings / groundAt
  triple) makes a later call safe and non-repeating.
• pendingCollidersRef already stashes landmark colliders that arrive before the controller
  exists and drains them when it is built — that path was added for [diorama-walk] SketchUp landmarks and the bridges have no collision — you walk and ride straight through them #702 and is covered by
  tests, so deferring cannot silently lose colliders.

The risk to respect

Entering walk must not now stall for 780 ms. If it does, the block has moved, not gone,
and the honest next step is chunking StaticWorld.build() rather than accepting a stutter
on mode switch. Measure the walk-entry cost as part of the fix.

Related

• /twins/[slug]?ortho freezes the main thread for 4.7s (17.8s total blocking) — the E2E click timeout is real #717 — the freeze this is the largest part of
• main has failed E2E on 8 consecutive runs — twins ?ortho overflow click, plus #715 #716 — umbrella: main red on E2E for eight straight runs