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 }

bindings/r/configure

@@ -13,6 +13,36 @@ set -e
 
 inc=""
 lib=""
+
+# WebAssembly (r-universe / webR): there is no prebuilt wasm C ABI to download,
+# but the build image ships cargo (/usr/local/cargo/bin) and emscripten
+# (EMSDK on PATH), so build the C ABI staticlib from source for
+# wasm32-unknown-emscripten right here. Building it in the same image with the
+# same emscripten avoids any ABI/version mismatch a prebuilt lib would risk.
+# rayon (threads) is dropped via --no-default-features; the indicators are pure
+# computation, so the serial path is functionally identical.
+if [ "$(uname -s)" = "Emscripten" ]; then
+  version=$(sed -n 's/^Version:[[:space:]]*//p' DESCRIPTION)
+  echo "wickra: building C ABI from source for wasm32-unknown-emscripten (v${version})"
+  build=$(mktemp -d)
+  url="https://github.com/wickra-lib/wickra/archive/refs/tags/v${version}.tar.gz"
+  "${R_HOME}/bin/Rscript" -e "download.file('${url}', '${build}/src.tar.gz', mode = 'wb', quiet = TRUE)" \
+    || { echo "wickra: failed to download source ${url}"; exit 1; }
+  "${R_HOME}/bin/Rscript" -e "untar('${build}/src.tar.gz', exdir = '${build}')"
+  root="${build}/wickra-${version}"
+  rustup target add wasm32-unknown-emscripten 2>/dev/null || true
+  ( cd "${root}" && cargo build -p wickra-c --release \
+      --target wasm32-unknown-emscripten --no-default-features ) \
+    || { echo "wickra: cargo wasm build failed"; exit 1; }
+  cp "${root}/bindings/c/include/wickra.h" src/wickra.h
+  cp "${root}/target/wasm32-unknown-emscripten/release/libwickra.a" src/libwickra.a
+  rm -rf "${build}"
+  # The static archive is linked into the package object; no shared lib to
+  # bundle and no rpath needed.
+  sed "s|@WICKRA_RPATH@||" src/Makevars.in > src/Makevars
+  exit 0
+fi
+
 if [ -n "${WICKRA_INCLUDE_DIR}" ] && [ -n "${WICKRA_LIB_DIR}" ]; then
   echo "wickra: using C ABI from WICKRA_INCLUDE_DIR / WICKRA_LIB_DIR (dev override)"
   inc="${WICKRA_INCLUDE_DIR}"