Address review feedback on Phase 4 underwater medium tracking without expanding scope

[Copilot]

This stacked PR resolves review feedback on the Phase 4 underwater-medium work while keeping the editor_params host/shader mismatch explicitly out of scope (it is pre-existing from PR #201). The branch focuses on correctness of water-state seeding/layout and clarifies shader behavior boundaries.

• Host-side water-plane classification fix

  • Corrected camera-in-water detection to use the engine’s plane convention dot(n, p) + d = 0.
  • Replaced the previous comparison with signed-distance logic so non-zero-height water planes seed water_state.x correctly.

• Push/cbuffer alignment intent preserved

  • Kept water_state at the intended offset for Phase 4 medium tracking.
  • Did not fold in unrelated editor_params host-struct expansion in this PR.

• Shader documentation cleanup (behavioral scope)

  • Updated in_water comments to match implementation: Phase 4 toggles on MAT_WATER refract path only.
  • Removed misleading mention that dielectric path flips medium state.

// Engine-side camera-under-water check (plane: dot(n, p) + d = 0)
const float signed_dist =
    nx * cam.pos.x + ny * cam.pos.y + nz * cam.pos.z + d;
if (signed_dist <= 0.0f) { cam_in_water = true; }