The page restores a model's cooked declared bodies as world bodies

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Why

Split from #493 (~950 lines): once the compiler cooks declared bodies (#493), the page restores them as world bodies.

To do

• Declared bodies restored by the page: the page reads the cooked bodies of physics.json and restores them as world bodies through the one slot owner of fix(physics): physics steps whoever leads the frame; soft bodies moved and hidden without rebuilding; one owner per body slot (#740) #752; nothing is built in the browser.

Code context

#493's cooked bodies in physics.json, physics/bodySlots.ts (#752), #711's restore path for cooked soft bodies (reuse it).

Proof

Tests by behaviour: a model with declared bodies restores them with the cooked hull, mass, centre of mass and inertia; a removed model releases their slots.

Links

Blocked by #493. Split from #493.

Narrowed (lead physics, 28 Sept.: one issue = one PR)

This issue closes with five To-dos: declared centre without declared inertia, retry after a rescale refusal, one per-model bookkeeping, kinematic child of a dynamic body, collider node outside the subtree recorded. "Mesh children of a dynamic body collide" (~250 lines, a new native cook) moved to #354.

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To-dos added by lead physics (from #493's review):

• The hull turns about the cooked centre of mass: a cooked body's hull is placed so it turns about shape.cooked.mass.centerOfMass, not the hull's own centre (FORMAT.md bodies).
• A declared mass wins: what the node's motion declares (mass, centerOfMass, inertiaDiagonal) wins over the cooked mass; the cooked one is handed to Jolt as provided mass only when nothing is declared.

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CTO decision (recorded by lead physics) and design note:

• The hold: every declared body is restored now with its cooked hull, mass, centre of mass and inertia (or its declared ones, which win) as a slot owner of its model (fix(physics): physics steps whoever leads the frame; soft bodies moved and hidden without rebuilding; one owner per body slot (#740) #752). A dynamic declared body is held at its drawn pose — created kinematic and asleep — so nothing lies: the collider stays where the node is drawn. A kinematic declared body is driven by the page as any kinematic body.
• Release condition: a held body becomes dynamic the moment its node can be drawn moving (Compiled dynamic bodies move their drawn nodes under physics #432's moving compiled nodes); one flag in the restore path, flipped by Compiled dynamic bodies move their drawn nodes under physics #432, nothing else changes.
• Switch from ground: a declaring node leaves the static instances once its body is restored (the additive Compiled models bring their moving bodies: declared motion, cooked hulls and mass #493 cook keeps both; the page takes the body and skips the instance), so no collider is doubled.
• Budget: bodies and triangles count against budget.physics like tiles. Paths: page physics only (restore through feat(physics): the page restores a compiled model's cooked soft bodies, nothing built in the browser (#699) #711's cooked path and bodySlots.ts); no draw path. Scenes: unit tests on the streamed-model fixture — a model with a declared dynamic box (held, kinematic, at its pose), a shapeless node restoring its cooked hull and provided mass (the hull turning about the cooked centre of mass), a declared mass winning, a removed model releasing the slots, and no doubled collider with the static instance.

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taken by lead physics

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Deviations from the design note (lead physics), before the merge: (1) a declared body whose collider is on another drawn node keeps that node's static instance beside the held hull — physics.json does not record the collider node, so the page cannot skip it; both stand where they are drawn while the body is held, so nothing lies; the fix (a compiler and format change) is a To-do on #432, where the hold is released. (2) A cooked hull counts 0 triangles against budget.physics.triangles, as a page-built hull does (a hull is not a triangle mesh); bodies count against the body budget.