feat(physics+combat): window-gap passability + shoot-through

[Chemaclass]

📚 Description

The 4/4 sub-issue that completes epic #375 Phase 3 (variable ceilings): a lowered ceiling now blocks movement and shots, not just the eye. Behaviour-neutral on every shipped (flat-ceiling) level.

🔖 Changes

• Physics try-move gains a headroom gate: a cell is walk-through only when its opening ceiling-z - floor-z clears player-height (= one fz-step, 0.25). A pinched lintel blocks walking but stays see-through (upper slice still paints, feat(render): upper wall-slice painter + ceiling-cast row distance #387). player-height = fz-step keeps L2's fz-0.75 platform (opening 0.25) walkable.
• Combat hitscan routes through new cast-shot-dist (gap-aware cast-ray): a floor cell whose ceiling dropped to/below the muzzle height (0.5 + player z) stops the shot; higher ceilings are shoot-through. Floor risers do not block shots, so tiered-floor levels are unchanged.
• Deterministic physics + engine unit tests pin block/pass/neutral cases; docs + CHANGELOG updated. Enemy chase headroom parity deferred (no shipped lowered-ceiling content), consistent with the epic's other deferred enemy-elevation work.

Review: clean-code-reviewer approved, no blocking findings.

Closes #388