feat: compile the snapshot fixtures' generated output in CI

[owenthcarey]

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:

Target	Check
C	clang -fsyntax-only -x c weaveffi.h
C++	clang++ -std=c++17 -fsyntax-only -x c++ weaveffi.hpp
Python	python3 -m py_compile on every .py
Node	node --check index.js; tsc --noEmit --strict --types node types.d.ts
Wasm	node --check weaveffi_wasm.js; tsc --noEmit --strict weaveffi_wasm.d.ts
Swift	swiftc -parse on Sources/WeaveFFI/*.swift
Go	go vet with the C header copied alongside (cgo needs it)
Dart	dart pub get && dart analyze
Kotlin	kotlinc with kotlinx-coroutines on the classpath
.NET	dotnet build
Ruby	ruby -c on every .rb

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.