feat: add Emscripten support to the WASM generator

[owenthcarey]

Summary

• Adds an emscripten flag to the WASM generator config. When set, loadWeaveffiWasm accepts a pre-initialized Emscripten Module (or the promise returned by its MODULARIZE factory) instead of a .wasm URL. The loader prologue binds the module's underscore-prefixed exports to the symbol names the glue calls (quoted bracket access for Closure Compiler safety), with a live get memory() getter over HEAPU8 so memory growth is handled. Everything after the prologue is byte-identical to the standard loader; no standard-mode .js snapshot changed.
• Cherry-picks the cabi.rs visibility change from the fork: {PREFIX}_API now expands to __attribute__((used, visibility("default"))) under __EMSCRIPTEN__ (the EMSCRIPTEN_KEEPALIVE expansion), so exports survive dead-code elimination without an -sEXPORTED_FUNCTIONS list.
• Closes the allocator ABI gap instead of substituting _malloc/_free: the generated C header now declares {prefix}_alloc / {prefix}_dealloc alongside the other runtime helpers, and the {prefix}.c scaffold provides malloc/free-backed defaults.
• Fixes the .d.ts byte type from the Node-only Buffer to Uint8Array.
• Async functions in Emscripten mode become explicit throwing stubs and are omitted from the .d.ts (the same treatment listeners get); documented in the generated README and the book.

Credit

Based on the design validation by @ariankordi, who proved out Emscripten support end to end in the emscripten-support fork and reported it in #22. This PR implements the loader through a config flag plus a load-time export adapter rather than per-call-site branching, but the cabi.rs visibility fix and the overall validation come from that work.

Closes #22.

Test plan

• 7 new unit tests in weaveffi-gen-wasm: adapter loader shape, prefixed runtime bindings, byte-identical shared helpers between modes, async stubs, .d.ts loader signature and async omission, Uint8Array mapping
• New CLI end-to-end test for inline generators.wasm.emscripten: true
• cargo test --workspace passes; the 32 changed snapshots (C/C++ headers, weaveffi.c, two .d.ts) reviewed and accepted
• cargo fmt --check and cargo clippy --workspace --all-targets -- -D warnings clean
• Generated Emscripten-mode JS parses and imports under Node; generated weaveffi.c compiles with -Wall -Wextra -Werror