fix(renderer): cull splats by projected footprint

[yingliang-zhang]

Summary

• replace center-only lateral clipXY rejection with conservative projected-footprint culling for standard 3D Gaussians
• derive the NDC footprint bound from the final ellipse axes after covariance projection, blur, standard-deviation scaling, focal adjustment, and pixel-radius clamping
• keep the 2DGS direct-corner path conservative by relying on GPU primitive clipping instead of rejecting by center
• document the updated clipXY semantics and add a focused shader source-contract regression

Motivation

The vertex shader currently rejects a splat when its center is outside the lateral clipXY boundary. A large or anisotropic Gaussian can still overlap the retained region even when its center is outside that boundary, so the center-only test can remove visible border contributions.

This change waits until the projected covariance and final ellipse axes are available. It computes the ellipse's axis-aligned NDC extent and rejects only when the complete footprint misses the configured boundary. A one-projected-pixel padding with a small NDC floor keeps the comparison conservative at float32 boundaries.

Scope

This is the lateral X/Y part of the broader clipping discussion in #226. It intentionally leaves the existing behind-camera and near/far center tests unchanged; precise Gaussian truncation across near/far planes also interacts with sorting and requires a separate design.

For 2DGS, the existing direct-corner path has no equivalent projected-covariance bound. It now defers lateral clipping to normal GPU primitive clipping rather than retaining the old false-negative center test. The tradeoff is additional vertex work for off-screen 2DGS primitives.

Verification

• npm run build:wasm
• npm test — 2/2 passed
• npm run lint — 85 files checked, no findings
• npm run build — production and development bundles built successfully
• real WebGL browser capture exercised the shader with no compilation or console errors

Related to #226; this PR does not close the near/far clipping request.