[Based on #2082] fix(runtime): decouple wasm from native host services

[cpunion]

Depends on #2082

Summary

• default raw GOOS/GOARCH wasm builds to the supported nogc runtime path
• keep wasm timer, signal, unwind, symbol lookup, and platform constants off native host implementations
• stop wasm from compiling or linking libuv, BDWGC, and native fault/unwind wrappers
• add a gating CI matrix for js/wasm and wasip1/wasm with exact Go 1.24.2 and Go 1.26.5 toolchains
• build llgo with the pinned primary Go toolchain before switching to the compatibility toolchain under test
• preserve existing native and named-target selections

This PR remains stacked on #2082 so the CI job validates the complete wasm fix. After #2082 merges, its source-adaptation diff disappears from this PR.

Fixes #2080.

Validation

• macOS arm64, Go 1.26.5:
  • js/wasm and wasip1/wasm builds pass without an explicit nogc tag
  • internal/build: 73.5% statement coverage
• Linux amd64, Emscripten 4.0.21:
  • Go 1.24.2: js/wasm and wasip1/wasm builds pass
  • Go 1.26.5: js/wasm and wasip1/wasm builds pass
  • internal/build: 72.8% statement coverage
• all artifacts are valid WebAssembly modules; inspected artifacts contain no uv_* or GC_* symbols

The issue reproduction is a build failure. Executing raw GOOS artifacts still requires the existing host-runner support for Emscripten imports on js and env.longjmp/shared memory on wasip1, which is outside this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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