bindings/r: build the C ABI from source for the WebAssembly target (#244)

## 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.

bindings/c/Cargo.toml

@@ -42,5 +42,18 @@ cast_sign_loss = "allow"
 similar_names = "allow"
 float_cmp = "allow"
 
+[features]
+# `parallel` (rayon-backed batch in wickra-core) is on by default for native
+# builds. The wasm32-unknown-emscripten target has no threads, so the R
+# package's wasm build (r-universe / webR) compiles this crate with
+# --no-default-features to drop rayon; wickra-core falls back to its serial
+# batch path, which is cfg-gated behind the same feature.
+default = ["parallel"]
+parallel = ["wickra-core/parallel"]
+
 [dependencies]
-wickra-core = { workspace = true }
+# Direct path dep rather than `workspace = true`: a member-level
+# `default-features = false` is ignored when inheriting a workspace dep that
+# does not set it, which would leave rayon in the wasm build. wickra-c is
+# `publish = false`, so no version pin is needed (and none to keep in sync).
+wickra-core = { path = "../../crates/wickra-core", default-features = false }