C ABI hub crate (bindings/c) foundation (#222)

## 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.

examples/README.md

@@ -21,6 +21,19 @@ The Rust examples live in the `wickra-examples` workspace member crate.
 | `strategy_macd_adx.rs` | Hourly BTCUSDT trend-follower: MACD crossover entries gated by ADX(14) > 20. | `cargo run --release -p wickra-examples --bin strategy_macd_adx` |
 | `strategy_bollinger_squeeze.rs` | Daily BTCUSDT Bollinger-squeeze breakout with ATR(14) trailing stop. | `cargo run --release -p wickra-examples --bin strategy_bollinger_squeeze` |
 
+## C / C++ — `examples/c/`
+
+Build the library first (`cargo build -p wickra-c --release`), then build and run
+the examples via CMake:
+`cmake -S examples/c -B examples/c/build -DWICKRA_LIB_DIR="$PWD/target/release"` →
+`cmake --build examples/c/build` → `ctest --test-dir examples/c/build`.
+
+| Example | What it does | CMake target |
+| --- | --- | --- |
+| `smoke.c` | Links the generated header + library and asserts SMA streaming / batch values across the boundary. | `smoke` |
+| `streaming.c` | Feed a tick stream through an EMA, printing each value (NaN during warmup). | `streaming` |
+| `smoke.cpp` | C++ RAII via `wickra::Handle` from [`wickra.hpp`](../bindings/c/include/wickra.hpp): construct, move, auto-free. | `cpp_smoke` |
+
 ## Python — `examples/python/`
 
 | Example | What it does | Run |