feat: compile the snapshot fixtures' generated output in CI

Problem

The snapshot suite (crates/weaveffi-cli/tests/snapshots.rs) pins the exact text every generator emits for five fixtures, and the conformance harness (conformance/run.sh) compiles and runs real consumers for the seven samples/. Neither checks that the fixtures' output compiles.

That gap matters because the fixtures reach shapes the samples do not: reserved-word identifiers, [T?]? nesting, mutable: true, iter<string>, type-level deprecated, async returning structs, and so on. During #44 a one-off manual pass that fed the edge_cases fixture through each language's compiler or analyzer found seven latent bugs that had shipped in every prior release (a Python SyntaxError, a C header that broke under C++, an invalid C# array creation, a Dart type shadowed by a field, a Wasm .d.ts that failed tsc --strict, and two local-variable collisions). Snapshots alone would have recorded all of them as "expected output".

Proposed solution

Add a CI step that, for every fixture under crates/weaveffi-cli/tests/fixtures/, generates all eleven targets and runs a syntax or type check per language, with no producer library required:

The conformance job already installs all of these toolchains, so this fits as a new phase in conformance/run.sh (or a sibling script it calls) rather than a new workflow. Failures should name the fixture, target, and file.

Alternatives considered

• Rely on snapshot review. This is the status quo; it depends on a reviewer spotting None = 0 in a 2,000-line diff.
• Add every edge shape to a sample. Samples need a real Rust producer for each function, which is a lot of boilerplate for shapes that exist only to stress the generators.
• Write per-language compile tests in Rust. Possible via std::process::Command, but the toolchain setup already lives in the conformance job and belongs there.

Target(s)

• C
• C++
• Swift
• Android / Kotlin
• Node.js
• WebAssembly
• Python
• .NET
• Dart
• Go
• Ruby
• CLI / core / IR / docs / other

Additional context

Once this lands, #45 (typed handles) will fail the Dart check for kitchen_sink and edge_cases until it is fixed, which is the intended behavior: the fixture keeps the broken shape so the check enforces the fix. Known pre-existing go vet noise ("possible misuse of unsafe.Pointer" on the uintptr-to-pointer pattern) will need either a fix in the Go generator or a -unsafeptr=false flag on that lane.

Start with conformance/run.sh and the fixture generation used by crates/weaveffi-cli/tests/snapshots.rs; run the existing conformance job to understand its phases and available toolchains. Add compilation or analysis checks for every fixture and target listed in the issue, with failures identifying the fixture, target, and file, then verify the full conformance job passes apart from documented go vet noise.