fix: render non-indexed geometry

[besmpl]

What

• render geometries with nil or empty indices using non-indexed Draw
• validate the vertex count before converting it to uint32
• preserve vertex-buffer caching and the existing indexed draw path
• document the nil/empty index contract and cover both paths with noop-HAL integration tests

Why

Geometry.Indices already permits non-indexed geometry, but the renderer always created and bound an index buffer and issued DrawIndexed. Custom non-indexed geometry therefore failed before drawing.

Impact

Non-indexed geometry now renders through the public renderer contract without allocating or caching an index buffer. Indexed geometry behavior is unchanged.

Verification

• go test ./...
• go test -race ./...
• focused draw tests repeated under the race detector
• go build ./...
• go vet ./...
• Linux, Windows, and WebAssembly cross-builds
• golangci-lint v2.12.2
• go mod verify
• gofmt and git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[kolkov]

Code reviewed: correct branching for non-indexed geometry (Draw vs DrawIndexed). Vertex count validation covers negative + overflow. Enterprise-grade test infrastructure with recording wrappers. 4 tests. LGTM.