Add collision layers with a per-project collision matrix

[Gopmyc]

Description

A project now holds up to 32 named collision layers and a symmetric matrix deciding which ones collide. Every physical object carries a layer, applied as the Bullet broadphase group and mask, so nothing filters collisions today beyond making a body a trigger or disabling it. Layers are edited in Project Settings, picked in the inspector, and both the layer and the matrix are readable and writable from Lua.

Related Issue(s)

Fixes #856

Review Guidance

• Layers are fixed slots, as in Unity : naming a free slot creates a layer, removing one frees its slot, and indices never shift. Scenes store the index in collision_layer, so renaming is free. A freed slot is reused by the next created layer, and objects left on it join it.
• 32 is the width of a Bullet collision mask, not an arbitrary cap.
• Layer and matrix changes are applied in PreUpdate, never synchronously. The contact callback reaches Lua inside stepSimulation, so removing a body from the world there would free the collision algorithm in use. Re-entering the world is also what drops the cached broadphase pair, whose manifold would otherwise keep pushing filtered bodies apart.
• Raycasts force AllFilter, so they keep hitting every layer.
• Static bodies no longer get Bullet's implicit StaticFilter exclusion, so static pairs now reach the broadphase and are rejected by needsCollision, both bodies being inactive. More pairs, same behaviour.
• New project keys collision_layer_<n> and collision_mask_<n>, left out of IsProjectSettingsIntegrityVerified on purpose : that check wipes every setting when a key is missing. Existing projects keep their settings and start with the Default layer alone, and scenes without collision_layer load on it.
• Out of scope : ignoring collisions between two specific objects, and a layer mask parameter on Physics.Raycast.

Screenshots/GIFs

N/A

AI Usage Disclosure

Refactored code

Checklist

• My code follows the project's code style guidelines
• When applicable, I have commented my code, particularly in hard-to-understand areas
• When applicable, I have updated the documentation accordingly
• My changes don't generate new warnings or errors
• I have reviewed and take responsibility for all code in this PR (including any AI-assisted contributions)