C ABI: full example suite + docs & About coverage (#224)

Stacked on #222 (base `feat/c-abi-hub`), so the diff is just the additions on top of the hub foundation — no merge of #222 required.

## What this adds

**Examples — full parity with rust/python/node (`examples/c/`)**
- `streaming.c` upgraded to the multi-indicator (SMA/EMA/RSI/MACD + signals) demo
- `backtest.c`, `multi_timeframe.c` (manual time-bucket resampling), `parallel_assets.c` (serial vs OpenMP fan-out, one handle per asset)
- three educational strategies: `strategy_rsi_mean_reversion.c`, `strategy_macd_adx.c`, `strategy_bollinger_squeeze.c`
- two network examples shelling out to `curl`: `fetch_btcusdt.c`, `live_binance.c` (REST poll)
- two header-only helpers (`wickra_csv.h`, `wickra_strategy.h`) since the C ABI ships no IO layer
- CMake builds all 11; the 9 offline ones run under `ctest` on 3 OS; the network two are built-only

**Docs & metadata — surface the C ABI everywhere it was missing**
- ARCHITECTURE diagram + crate table, SECURITY + THREAT_MODEL (the C ABI as the sole `unsafe` FFI surface), the three binding package READMEs, issue/PR templates, CHANGELOG, and the GitHub About template (live About + org description updated too)

**Cleanup**
- removed all references to the private generator tooling from public files (`bindings/c/src/lib.rs` header, `CONTRIBUTING.md`, `sync-about.yml`)

Verified locally: `cargo build -p wickra-c --release`, `cmake + ctest` (9/9 pass), and `-Wall -Wextra -Wpedantic` clean on gcc 13.

ARCHITECTURE.md

@@ -29,14 +29,20 @@ or replace lives behind a separate crate boundary.
                 ▲
                 │ (every binding wraps the same core)
                 │
-   ┌────────────┴───────────┬─────────────────────┐
-   │                        │                     │
-┌──▼──────┐         ┌───────▼──────┐      ┌───────▼────────┐
-│ Python  │         │     Node     │      │     WASM       │
-│ (PyO3)  │         │ (napi-rs)    │      │ (wasm-bindgen) │
-└─────────┘         └──────────────┘      └────────────────┘
+   ┌───────────┬────────────┴┬──────────────┬──────────────────────┐
+   │           │             │              │                      │
+┌──▼─────┐ ┌───▼────┐ ┌──────▼──────┐ ┌─────▼──────────────┐
+│ Python │ │  Node  │ │    WASM     │ │  C ABI (cbindgen)  │
+│ (PyO3) │ │(napi-rs)│ │(wasm-bindgen)│ │  cdylib + header   │
+└────────┘ └────────┘ └─────────────┘ └────────────────────┘
 ```
 
+Python, Node and WASM are *native* Rust bindings (PyO3 / napi-rs /
+wasm-bindgen). The C ABI is the *hub* every other C-capable language links
+against: it builds to a `cdylib`/`staticlib` plus a generated `wickra.h`, and
+C / C++ / Go / C# / Java / R consume that one artifact rather than each
+re-wrapping the core.
+
 | Crate | Path | What it owns | Public deps |
 |---|---|---|---|
 | `wickra-core` | `crates/wickra-core` | every indicator, the `Indicator` trait, `BatchExt`, `Candle`/`Tick` types, `Error` | `thiserror`, `rayon` (parallel batch) |
@@ -45,6 +51,7 @@ or replace lives behind a separate crate boundary.
 | `wickra-python` | `bindings/python` | `_wickra` PyO3 module + Python package | `pyo3`, `numpy`, depends on `wickra-core` |
 | `wickra-node` | `bindings/node` | NAPI-RS native binding | `napi`, depends on `wickra-core` |
 | `wickra-wasm` | `bindings/wasm` | WebAssembly binding | `wasm-bindgen`, depends on `wickra-core` |
+| `wickra-c` | `bindings/c` | C ABI hub — `cdylib`/`staticlib` + generated `wickra.h` (cbindgen) | depends on `wickra-core` |
 | `wickra-examples` | `examples/rust` | runnable binary examples | depends on `wickra`, `wickra-data` |
 
 The `fuzz/` directory is **excluded** from the workspace (it has its own