Iso 2.0 [4/12]: Negative-Z Carve-out Rendering (river, river-bank)

[putersdcat]

Spec reference

IsoRenderingPlanV2.1.md §3.5 Negative-Z Nano; §Addendum B §2

Goal

Negative-Z nanos render as sunken depressed tiles — the tile surface appears
below the surrounding ground level. River water texture is visible at depth.
River-bank shows a transitional blended edge.

Technical requirements

• Flat iso projection (same matrix as base tiles) shifted down by sinkPx = abs(zOffset) × Z_PX_PER_LEVEL.
• Entire render clipped to parent tile diamond (clipDiamond).
• blendEdges=true → four-sided inward gradient overlaid on tile edges.
• Sink amount: zOffset=-2 → 8px sink.

Acceptance criteria (ALL required to close)

• render_nano_tile river straight-h → water texture visible, clearly below ground plane, edge blending on all 4 diamond sides.
• render_nano_tile river cross → 4-way river crossing, continuous water surface.
• render_nano_tile river-bank straight-h → transitional bank tile (partial water, partial land blend).
• In a mixed scene: river tiles sit visually lower than surrounding grass tiles.
• PNGs committed: ProgressEvaluations/nano-river-{variant}.png for at least 4 variants + scene-river-in-grass.png.

Verification command

render_nano_tile kind=river variant=straight-h outputPath=ProgressEvaluations/nano-river-straight-h.png
render_nano_scene entries=[... grass terrain + river row ...] outputPath=ProgressEvaluations/scene-river-in-grass.png