Add 16-bit noise heightmap export with per-pixel 3D detail

[raguilar011095]

Summary

• Adds a new Noise Heightmap (16-bit) option to the export dropdown that layers CPU-side 3D simplex noise on top of the GPU-rendered base heightmap, producing fractal-rich terrain suitable for downstream detailing in external tools.
• New js/noise-heightmap.js samples noise on the unit sphere (no equirectangular distortion) and combines domain-warped multi-band FBM, ridge noise on mountain spines, slope-aligned anisotropic detail, sedimentary banding on plateaus, a lowland quieting mask, and elevation/slope-driven erosion carving.
• Amplitude factors are max-pooled then Gaussian-blurred with sphere-aware (latitude-corrected) kernels at fixed physical radii (~50 km) so detail bleeds smoothly between mountains and lowlands without resolution-dependent artifacts.

Test plan

• Open the app, generate a planet, select Noise Heightmap (16-bit) from the export dropdown and confirm a orogen-noise-heightmap-<seed>.png downloads
• Inspect the exported PNG: verify mountains have ridged spines, plateaus show subtle banding, and lowlands have varied (not uniform) flat plains
• Compare against the existing Heightmap (B&W) export at the same seed to confirm large-scale terrain shape is preserved
• Re-run the export at multiple Detail-slider settings and confirm the noise character looks equivalent across resolutions (scale invariance)
• Verify the existing heightmap, landheightmap, and landmask exports still produce identical output to before

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