Function applyLensDistortion

Apply Brown–Conrady distortion to UV coordinates and write the result into out. Returns out for ergonomic chaining.

Y-convention contract (MED-27): this function IS the canonical distortion map, defined in TOP-DOWN uv space — the picking-system's screen convention (event.clientY / height, y=0 at the top). The TSL mega shader consumes it natively (WebGPURenderer's fullscreen triangle delivers top-down uv). The GLSL mega shader runs only under WebGLRenderer, whose fullscreen triangle delivers BOTTOM-UP uv — it therefore applies the exact y-flip conjugation internally by negating the two odd-symmetry intrinsics (principalPoint.y and skew) in its K matrix rather than flipping uv. Radial/focal terms are flip-even, which is why the historical mismatch was invisible with the default (ppy = skew = 0) presets and why textual-identity parity tests could not catch it. The conjugation-identity test in lens-distortion.test.ts pins the relationship: do NOT "re-sync" the GLSL matrix verbatim to this formula.

Algorithm (must stay in sync with mega/shader.glsl.ts::applyDistortion):

1. UV ∈ [0, 1] → normalised n ∈ [-1, 1].
2. Radial distortion: xd = (1 + k_x · r²) · xn (likewise y).
3. Apply intrinsic matrix K with skew, principal point, focal length.
4. Map back into UV ∈ [0, 1].