improve: only require scene-color mips for non-zero LOD

[odvcencio]

Summary

Refines the SceneColorMips requirement so the host only builds a scene-color mip chain when sampling at a non-zero or dynamic LOD. Level-zero taps continue to emit textureSampleLevel for WGSL non-uniform control flow compatibility, but skip unnecessary mip generation. Also implements missing interface stubs in IR test helpers to satisfy updated contract expectations.

Changes

• Introduce StageRequiresSceneColorMips and RequiresSceneColorMips to distinguish explicit LOD usage from actual mip chain dependencies
• Update hostRequirements to conditionally set SceneColorMips based on whether the sampled LOD is non-zero or dynamic
• Add isLiteralZero helper to safely evaluate literal expressions during LOD analysis
• Clarify documentation for SceneColorMips and SceneSampleLevel regarding level-zero tap behavior
• Fulfill newly added interface methods (exprs, children, nestedStmts) on unrenderable IR test types

Testing

• Run go test ./... to verify all tests pass, including the new scenarios in lower/post_surface_test.go