Validation: collider volume vs. source-mesh volume sanity check

[Weisl]

Context

Cross-engine research (Unity/Unreal/Godot official docs) found that authored collider volume/shape has real, documented simulated-physics consequences beyond collision detection, in all three target engines:

• Unreal computes simulated mass directly from collider volume: Physical Material Density (g/cm³) x the simple collision shape's volume = mass, by default (Override Mass available to bypass this).
• Unity and Godot keep mass manual, but both auto-compute center of mass and/or inertia tensor from the actual attached collider geometry by default - affecting tumbling, toppling, and torque-response realism even though raw mass doesn't change.

This means a degenerate or badly-fitted collider (e.g. a leftover default box, or a failed decomposition that produced something wildly oversized/undersized relative to the source mesh) doesn't just collide wrong on import - in Unreal it gives the object the wrong simulated weight, and in Unity/Godot it skews computed inertia/center of mass.

Nothing in the addon currently checks for this class of problem - existing validation checks (check_triangle_count, check_too_many_colliders, etc.) don't compare a collider's volume to the render mesh it's meant to represent.

Proposal

Add a new validation check (alongside the existing checks in validation/checks.py) that compares a collider's volume against its source render mesh's own bounding/convex-hull volume, flagging results outside a reasonable configurable ratio (e.g. more than Nx larger or smaller). Natural sibling to #666 (convex-hull vertex-count validation).

Acceptance criteria

• New check appears in Validation panel results with a clear message (e.g. "collider volume is Nx [larger/smaller] than source mesh").
• Ratio threshold is configurable in Preferences, matching the existing validation threshold pattern.
• Test coverage alongside existing validation check tests.