fix: PT final upsample is phase-aware (bilinear + depth joint bilateral)

[proggeramlug]

The trace-to-full upsample mapped every output pixel to its CONTAINING
trace texel by integer division and centred the kernel weights there -
no fractional phase. The lighting therefore reconstructed as
trace-texel-sized constant blocks, and because the trace grid rotates
its sample phase every frame (so the EMA integrates all four sub-
positions), the block boundaries CRAWLED whenever the camera moved.
On top of renderScale 0.5 (trace = quarter of output res) that crawl
was the residual pixelated-while-moving look indoors after the
PT-9/9b disocclusion fixes.

The taps now weight by tent distance from the CONTINUOUS trace-space
position (texel centres aligned), turning the pass into a true
bilinear-plus-depth joint bilateral: smooth gradients, stable under
the per-frame phase rotation, same 3x3 tap cost. The thin-geometry
epsilon fallback and the sky guard are unchanged, and the
full-res-trace path still degenerates to modulate-and-write.

Verified: both golden PT tests pass; the translating-dolly capture at
renderScale 0.5 shows smooth wall gradients with no block crawl at
unchanged fps.

Claude-Session: https://claude.ai/code/session_018574BdCfSjdpNK3WLgx1hP

Summary by CodeRabbit

• Bug Fixes
  • Improved image upsampling in the realtime rendering pipeline.
  • Reduced sampling artifacts and improved visual smoothness when reconstructing final rendered results.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 1bca2aa6-8257-42ac-9073-bb5c7bfaee34

📥 Commits

Reviewing files that changed from the base of the PR and between 491d9de and efe21f8.

📒 Files selected for processing (1)

• native/shared/src/renderer/shaders/pt.rs

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📝 Walkthrough

Walkthrough

The SVGF final pass replaces integer trace-texel mapping and B3-spline sampling with center-aligned sub-texel coordinates and tent weights. Depth-based weighting and surrounding bounds checks remain unchanged.

Changes

SVGF final-pass resampling

Layer / File(s)	Summary
Sub-texel coordinate mapping and tent weighting native/shared/src/renderer/shaders/pt.rs	cs_final computes center-aligned continuous trace coordinates with fractional offsets, then combines separable tent weights with the existing depth agreement term.

Estimated code review effort: 2 (Simple) | ~10 minutes

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/pt-upsample-phase

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