Emit optional binary SPIR-V (.spv) via SPIRV-Tools

[NripeshN]

Summary

The Vulkan target previously emitted SPIR-V only as text (.spvasm). This adds optional binary SPIR-V (.spv) emission — real, GPU-loadable output — by assembling the generated assembly in-process with the SPIRV-Tools library.

• Optional dependency (mirrors the existing optional MLIR path): find_package(SPIRV-Tools CONFIG QUIET); when present, CROSSGL_HAVE_SPIRV_TOOLS=1 is defined and the library linked. A build without SPIRV-Tools still succeeds and simply emits only .spvasm (verified via -DCMAKE_DISABLE_FIND_PACKAGE_SPIRV-Tools=ON — cglc links, feature disabled).
• Assembler (SPIRVAssembler.h/.cpp): assembleVulkanSpirvText() uses the SPIRV-Tools C API (spvTextToBinary) with SPV_ENV_VULKAN_1_2, matched to the backend's vulkan1.2 / SPIR-V 1.5 target. Returns std::nullopt (stub) when the library is absent, so callers compile either way.
• Emission: the Vulkan backend writes a sibling <module>.spv next to the .spvasm (the package metadata already recognizes .spv). On assembly failure it surfaces the exact SPIRV-Tools diagnostic rather than hiding it.

Verification

• Builds with SPIRV-Tools found (feature on) and with it disabled (stub path) — both confirmed.
• Full suite green: 100% tests passed, 0 tests failed out of 2453.
• A real emitted .spv validates: spirv-val --target-env vulkan1.2 <module>.spv → valid; SPIR-V magic word 0x07230203; byte-identical to spirv-as assembling the same text.
• New spirv-val-gated smoke test (cglc_build_vulkan_spirv_tools_binary_smoke) that asserts the .spv exists, carries the magic word, and validates.

Follow-up

Native binary encoding (no SPIRV-Tools dependency) and wiring .spv into the .cglb package archive remain future work.

[NripeshN]

Closing: on review this is redundant. The Vulkan native path (buildVulkanPrototypeBinary) already produces a validated binary .spv via the spirv-as/spirv-val CLI, and this PR's in-process library write runs only after the spirv-as requirement check (so only when the CLI is present) and is overwritten byte-identically by the CLI command. The genuinely-additive change — making vulkan native builds self-contained (assemble + validate in-process via the linked SPIRV-Tools library, relaxing the spirv-as/spirv-val CLI requirements) — is a separate focused change; will revisit.