Per-binding throughput benchmarks + test-coverage gaps (#246)

Adds a `throughput` benchmark to every target and closes two small
test-coverage documentation/QA gaps. One PR, no merge of binding code beyond
the additive benchmarks and one C test.

## 1. Per-binding throughput benchmarks (all 9 targets)

Each benchmark feeds a deterministic synthetic OHLCV series through three
indicators chosen by **FFI call-signature archetype** (not algorithm — the same
Rust core runs underneath all bindings):

- `SMA(20)` — 1-in → 1-out (baseline boundary cost)
- `ATR(14)` — multi-in → 1-out (input marshalling)
- `MACD(12,26,9)` — 1-in → multi-out (output marshalling)

Streaming is timed for all three; batch for the single-output SMA and ATR
(median of 3 runs, after a warmup pass).

New: Python (PyO3), WASM, C (CMake), C# (Stopwatch), Go, Java (FFM), R, and the
Rust core baseline (`examples/rust/.../throughput.rs`, **no FFI** — the ceiling
the bindings are measured against and the value their batch paths converge
towards). Node already had `throughput.js`.

**Not a speed claim:** there is no comparable streaming TA library for C, C#,
Go, Java, R or WASM to compare against, so these are raw per-binding throughput
numbers documenting each language's FFI overhead — see BENCHMARKS.md §3. The
"Wickra is fast" claim still lives in §1/§2 (Rust core + the Python/Rust
cross-library runs).

## 2. README `## Testing`: C# and C bullets

The section listed every layer except C# and C, even though both have suites.
Adds the two missing bullets.

## 3. C archetype ctest

`examples/c/archetypes.c` drives one indicator per FFI archetype through the
real C boundary (scalar + batch==streaming, multi-output, bars, profile, array
input) plus reset, invalid-parameter and NULL-safety — the C counterpart of the
Go/R/Java archetype suites. Runs on three OSes via the existing CMake/ctest.

## Notes

- Benchmarks are not CI-gated (manual-run scripts, like the existing
  `throughput.js`); no `ci.yml`/`release.yml` changes.
- Docs: BENCHMARKS.md §3, a `## Benchmark` section in every binding README, a
  CHANGELOG entry.
- Verified locally by running: Rust, Python, C, C#, Go, Java (real numbers); the
  C archetype ctest with `-Wall -Wextra -Wpedantic -Werror`. WASM and R are
  API-correct and syntax-checked but need their own toolchains to run.

CHANGELOG.md

@@ -5,6 +5,20 @@ All notable changes to Wickra are documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+### Added
+- **Per-binding throughput benchmarks** — every target now ships a `throughput`
+  benchmark mirroring the Node `throughput.js`: streaming and batch
+  updates-per-second for `SMA(20)`, `ATR(14)` and `MACD(12,26,9)` over a
+  synthetic OHLCV series. New for Python (`bindings/python/benchmarks/`), C
+  (`bindings/c/benchmarks/`), C# (`bindings/csharp/benchmarks/`), Go
+  (`bindings/go/benchmarks/`), Java (`bindings/java/benchmarks/`), R
+  (`bindings/r/benchmarks/`), WebAssembly (`bindings/wasm/benchmarks/`) and the
+  Rust core baseline (`examples/rust/.../throughput.rs`, no FFI). They measure
+  each binding's FFI overhead — the same Rust core runs underneath all of them —
+  and are documented in [BENCHMARKS.md](BENCHMARKS.md) §3, not a cross-library
+  speed claim.
+
 ## [0.8.2] - 2026-06-10
 ### Fixed
 - **R binding builds for WebAssembly** — `bindings/r/configure` now builds the