[RowDaBoat]

Summary

Follow-up to #98. Reworks how polyworld/waters finds shorelines, and tunes Gods of the Arena's lakes.

• Shore distance field (waters.nim): the per-pixel screen-space shore march is replaced by a distance field baked once per water layer from the terrain. The bake samples the visible ground (solid layers plus ground relief) at 4 samples per world unit over the water and an 8-unit margin; ground above the water's surface is land. An exact Euclidean distance transform (Felzenszwalb–Huttenlocher, columns then rows) stores each sample's distance to the nearest land in a filtered R16F texture. The water reads it once per pixel, so the shoreline is smooth, follows the visible bank, and no longer changes with the camera or rings units standing in the water. Fields are built lazily by drawWaterBody; buildShoreFields() rebuilds them after terrain edits. shoreHeight is removed.
• Shoreline strip and foam: the shoreline is a flat strip foamDistance wide with a soft outer edge. foamNoise (0–1) breaks it into foam: tileable noise flow-mapped toward the shore along the field's gradient (the exact direction to the nearest shore), with foamScale and foamSpeed. The shore fade still blends water and strip into the bank.
• Gods of the Arena:
  • Lakes use a tuned GotaLakeWater preset (deep turquoise (0, 0.52, 0.77), colour strength 0.54, foam scale 1.22 and speed 0.1, fade 0.96 / curve 1.97).
  • The lake list now takes its parameters from lakeWater each frame, so they can change at runtime.
  • -d:waterPanel adds a developer window (F4) with every water parameter, plus colour pickers (saturation/value square, hue bar, swatch and RGB sliders) for the water and shoreline colours. It isn't compiled into normal or web builds.

Merged onto the renderer-performance rework (captureScene, beginReflection and drawWaterBody called inline by GotA).

Testing

• tests/all.nim: CI tests passed
• Native builds: GotA (default, -d:waterPanel, -d:headless), Light vs Dark, Call to Adventure
• GotA web viewer built and played in headless Chrome (SwiftShader WebGL2): no shader, WebGL or runtime errors
• GotA native screenshots: smooth shoreline strip and foam along the lake banks; no rings around units in the water; water colour measured against the target

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