feat: seamless chunk boundaries via extended heightmap normals

[maxfelker]

Problem

Visible seams appeared at chunk boundaries due to incorrect lighting at chunk edges.

Root Cause

ComputeNormals used clamped sampling at boundary vertices:

sampleH := func(row, col int) float64 {
    row = max(0, min(resolution-1, row))  // clamps — wrong at edges!
    col = max(0, min(resolution-1, col))
    ...
}

At the right edge (col=128), the central difference sees hL = hR, giving zero horizontal gradient and a flat, incorrect normal. Heights are continuous across chunks (same world-space noise), but the lighting seam came from mismatched normals.

Solution: Extended Heightmap Normal Computation

Generate a (resolution+2)×(resolution+2) = 131×131 heightmap for each chunk that includes a 1-cell border sampled one spacing unit beyond the chunk boundary. Use this extended heightmap to compute normals — every edge vertex now has real cross-boundary gradient data, no clamping.

chunk (0,0) extended: [-4, 0, 4, ..., 508, 512, 516]  ← wx coordinates
                        ↑ border                  ↑ border
chunk (1,0) extended: [508, 512, 516, ..., 1020, 1024, 1028]
                        ↑

Right edge of chunk(0,0) normal = central_diff(h(508), h(516)) / (2×4)
Left edge of chunk(1,0) normal = central_diff(h(508), h(516)) / (2×4)

Identical — seam eliminated.

Changes

File	Change
wasm/terrain/heightmap.go	Add GenerateExtendedHeightmap(cx, cz, cfg)
wasm/terrain/normals.go	Add ComputeNormalsFromExtended(extHm, res, size, scale)
wasm/main.go	Use extended path in goGenerateChunk
wasm/terrain/heightmap_test.go	Extended heightmap size + inner match + border tests
wasm/terrain/normals_test.go	Cross-chunk consistency + unit length + size tests

Performance

~7% more noise evaluations per chunk (131²=17,161 vs 129²=16,641). No TypeScript API changes. No GPU buffer format changes.