## Problem
r-universe builds every package to WebAssembly (webR) in addition to the native platforms, calling `./configure --host=wasm32-unknown-emscripten`. `bindings/r/configure` only knew Linux/macOS/Windows and exited with `unsupported OS 'Emscripten'`, so the **WASM job was the single red check** on an otherwise fully-published r-universe build (all native platforms + deploy are green; the package installs fine everywhere).
## Fix
The r-universe wasm build image ships **cargo** (`/usr/local/cargo/bin`) and **emscripten** (`EMSDK` on PATH). So instead of needing a prebuilt wasm lib (which would risk an emscripten ABI/version mismatch), `configure` now detects the Emscripten host and **builds the C ABI staticlib from source** for `wasm32-unknown-emscripten` in-place — compiled with the image's own emscripten, so the ABI always matches. The static `libwickra.a` is linked into the package object (no shared lib, no rpath).
`wickra-core`'s rayon batch needs threads (absent on wasm), so the wasm build drops it via `--no-default-features`. `wickra-c` now takes `wickra-core` as a direct path dep with `default-features = false` plus a default `parallel` feature that re-enables it for native builds (a member-level `default-features = false` is ignored when inheriting a workspace dep — that was the trap).
## Validated locally
- `cargo build -p wickra-c` (default) → rayon present, builds.
- `cargo build -p wickra-c --no-default-features` (the wasm feature path) → rayon **gone**, builds.
- `cargo build --workspace`, clippy, fmt all clean; `configure` passes `sh -n`.
## Cannot be validated locally
No Rust/emscripten wasm toolchain here. The wasm build only runs in r-universe, and `configure` downloads the matching `v${version}` source tag — so this takes effect from the **first release that includes it** (the tagged source must contain the feature toggle). Open risks: whether the image has the `wasm32-unknown-emscripten` Rust target pre-installed (configure runs `rustup target add` best-effort) and the wasm build time. Worth one r-universe rebuild to confirm.
Not merging — review first.
kingchenc
6433c9b3de