fix(lp-gfx-wgpu): hoist struct/const declarations above generated prototypes

[Yona-Appletree]

Summary

• assemble_fragment_glsl spliced generated function prototypes above the authored GLSL, so any signature referencing a user-defined struct (Point make_point(float)) failed naga glsl-in with "Unknown type: Point", and files carrying their own forward declarations hit "Prototype already defined" duplicates.
• New hoist_declarations pass over the texture-lowered authored text: top-level struct definitions and const declarations move above the generated prototypes (both signatures and struct members may use consts as array sizes), and authored prototypes are stripped with newline-preserving blanking so diagnostics keep their line numbers.
• Stripping-and-regenerating was chosen over "skip generating a duplicate": skipping would assign the function's naga arena slot at the authored prototype's position — after every generated prototype — breaking the callee-first arena order its callers need.
• The test-side reference renderer (tests/util/reference.rs) used the same splice pattern and now shares the hoist.
• Corpus: removed the ~120 splice-rationale @unsupported(wgpu.f32) annotations across 23 files; 21 now pass on the GPU. Two files fail for distinct, genuine naga limitations and are re-annotated with accurate rationales: function/edge-const-out-error.glsl (glsl-in rejects the const in parameter qualifier) and function/access-lvalue-local-out-inout.glsl (WGSL forbids addressing a vector component; wgsl-out emits &v[i] for vector-lane out/inout args).

Validation

• 4 new unit tests in assembly.rs (hoist ordering, prototype stripping vs. globals, struct-returning prototypes, line-count preservation); cargo test -p lp-gfx-wgpu fully green including render-parity and differential suites (Metal adapter).
• Full wgpu.f32 filetest corpus: 5246/5246 files pass (5222 directives pass, 0 fail; net −103 unsupported).
• The 23 re-enabled files: 600/600 across all CPU targets (interp.f32, wasm.q32, rv32c/n/lpn.q32).
• just check and just test both pass.

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