refactor(webgpu): data-driven gating for WGSL built-in inputs

[mvaligursky]

Follow-up to #8733. Generalizes the WebGPU shader processor's gating of optional WGSL built-in inputs so that all stages are handled consistently, fixes a regression that produced invalid WGSL for shaders without varyings, and restructures the emission to be data-driven so adding a new built-in is a one-row change.

Fixes:

• Empty FragmentInput struct (invalid WGSL) for fragment shaders with no varyings and no pc* references (e.g. the gizmo-unlit shader). A sentinel built-in is now emitted only when the input struct would otherwise be empty.
• Detection now also recognises struct-field access (input.frontFacing, input.sampleIndex, ...), not only the pc* global form. Both forms are documented as supported in wgsl-specifics.md.
• Word-boundary regex (instead of String.includes) prevents false positives on identifiers containing the name as a substring.

Changes:

• New FRAGMENT_BUILTINS / VERTEX_BUILTINS tables describe each optional built-in (name, type, @builtin(...), public pc* global, optional feature flag, optional sentinel role).
• Vertex inputs vertex_index / instance_index are now gated symmetrically with the fragment built-ins (previously always emitted). Gating uses both pcVertexIndex / pcInstanceIndex and input.vertexIndex / input.instanceIndex.
• Sentinel fallback per stage (position for fragment, vertex_index for vertex) keeps input structs non-empty even when nothing else is referenced.
• `processAttributes` signature extended with `device`, `source`, `entryInputName` (internal only).
• `copyInputs` left to do its own `ENTRY_FUNCTION` match against the post-`renameUniformAccess` source, since cached match positions would be invalidated by source-modifying passes.

No public API changes — all changes are internal to the WGSL shader processor. Behaviour for existing engine shaders is unchanged (the few vertex chunks that reference `pcVertexIndex` / `pcInstanceIndex` continue to receive those built-ins; the rest now no longer carry two unused inputs).