## 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.
## What
Introduces `wickra-c` — a `cdylib` + `staticlib` that exposes the Rust core over a **C ABI**. This is the hub every C-capable language (C, C++, Go, C#, Java, R) links against, instead of re-wiring each indicator natively. The native Python/Node/WASM bindings are untouched; this is purely additive, for ecosystems without first-class Rust tooling.
## Scope (foundation slice)
This PR deliberately validates the **whole pipeline end to end with one indicator (SMA)** before scaling to all 514, so the CI / cross-OS / header-drift mechanics are proven green first.
- Opaque `*mut T` handles; `wickra_<ind>_{new,update,batch,reset,free}`.
- NaN sentinel for warmup / NULL handles; caller-owned batch buffers; every function NULL-safe.
- cbindgen generates and commits `bindings/c/include/wickra.h` with opaque handle typedefs.
- A C smoke example (`examples/c/`) links the header + compiled library and runs (CMake + ctest).
- A `c-abi` CI job builds the library and runs the smoke test on **Linux, macOS and Windows**, plus a header drift check on Linux.
## Notes
- The per-indicator FFI blocks are plain `#[no_mangle]` functions, **not** a macro: cbindgen cannot see macro-generated functions on stable Rust (macro expansion needs nightly), so the blocks are written literally and will be generated mechanically by the ScriptHelpers `capi` wrapper in a follow-up (same model as the committed-but-generated Node `index.js`).
- `bindings/c` cannot inherit the workspace `forbid(unsafe_code)` lint (the C boundary needs raw pointers), so it mirrors every workspace lint and only relaxes `unsafe_code`. The Rust core stays `unsafe`-forbidden.
## Follow-ups (separate PRs)
- ScriptHelpers `capi` generator + wire the scalar family (~235).
- Hand-written blocks for multi-output / custom-input / bars (~279).
- Docs consistency wave (README / docs / webpage: Python·Node·WASM·Rust → +C).
- Release wiring (native-lib matrix + header/lib GH-release assets) — gated.
kingchenc