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.

bindings/csharp/benchmarks/Program.cs

@@ -0,0 +1,101 @@
+// Throughput benchmark for the Wickra C# binding.
+//
+// Measures how many indicator updates per second the binding sustains, both
+// per-tick (streaming Update) and bulk (Batch), over a synthetic OHLCV series.
+// It is the C# counterpart of the Node throughput.js and the Rust criterion
+// benches: it benchmarks Wickra's own O(1) streaming engine across the
+// managed<->C-ABI boundary (there is no comparable streaming TA library on
+// NuGet to compare against), so the headline number is raw per-binding
+// throughput / FFI overhead, not a cross-library ratio.
+//
+// Three indicators are timed, chosen by FFI call-signature archetype rather
+// than algorithm: SMA (1-in -> 1-out), ATR (multi-in -> 1-out), and MACD
+// (1-in -> multi-out). Streaming is timed for all three; batch only for the
+// single-output SMA and ATR (multi-output batch is not exposed uniformly).
+//
+//   cargo build -p wickra-c --release
+//   dotnet run -c Release --project bindings/csharp/benchmarks            # 200k bars
+//   dotnet run -c Release --project bindings/csharp/benchmarks -- --bars 1000000
+
+using System.Diagnostics;
+using System.Globalization;
+using Wickra;
+
+// Deterministic, locale-independent number formatting for the report.
+CultureInfo.CurrentCulture = CultureInfo.InvariantCulture;
+
+int bars = 200_000;
+for (int i = 0; i < args.Length - 1; i++)
+{
+    if (args[i] == "--bars" && int.TryParse(args[i + 1], out var n) && n >= 1000)
+    {
+        bars = n;
+    }
+}
+
+// Deterministic synthetic OHLCV (no RNG, so runs are comparable).
+var open = new double[bars];
+var high = new double[bars];
+var low = new double[bars];
+var close = new double[bars];
+var volume = new double[bars];
+var timestamp = new long[bars];
+for (int i = 0; i < bars; i++)
+{
+    double mid = 100 + Math.Sin(i * 0.001) * 20 + i * 1e-4;
+    double c = mid + Math.Sin(i * 0.05) * 2;
+    close[i] = c;
+    open[i] = mid;
+    high[i] = Math.Max(c, mid) + 1.5;
+    low[i] = Math.Min(c, mid) - 1.5;
+    volume[i] = 1000 + (i % 97) * 13;
+    timestamp[i] = i;
+}
+
+double Mups(double ns) => bars / (ns / 1e9) / 1e6;
+
+// Median elapsed-ns over a few repetitions, after one warmup pass.
+double TimeNs(Action fn, int reps = 3)
+{
+    fn(); // warmup (JIT + cache)
+    var samples = new double[reps];
+    for (int r = 0; r < reps; r++)
+    {
+        long t0 = Stopwatch.GetTimestamp();
+        fn();
+        samples[r] = (Stopwatch.GetTimestamp() - t0) * (1e9 / Stopwatch.Frequency);
+    }
+    Array.Sort(samples);
+    return samples[reps / 2];
+}
+
+// SMA (scalar 1-in/1-out), ATR (multi-in/1-out), MACD (1-in/multi-out).
+var indicators = new (string Name, Action Stream, Action? Batch)[]
+{
+    ("SMA(20)",
+        () => { using var ind = new Sma(20); for (int i = 0; i < bars; i++) ind.Update(close[i]); },
+        () => { using var ind = new Sma(20); ind.Batch(close); }),
+    ("ATR(14)",
+        () => { using var ind = new Atr(14); for (int i = 0; i < bars; i++) ind.Update(open[i], high[i], low[i], close[i], volume[i], timestamp[i]); },
+        () => { using var ind = new Atr(14); ind.Batch(open, high, low, close, volume, timestamp); }),
+    ("MACD(12,26,9)",
+        () => { using var ind = new MacdIndicator(12, 26, 9); for (int i = 0; i < bars; i++) ind.Update(close[i]); },
+        null), // multi-output: streaming only
+};
+
+Console.WriteLine($"Wickra C# throughput - {bars:N0} bars (median of 3 runs)\n");
+Console.WriteLine($"{"Indicator",-22}{"streaming (Mupd/s)",20}{"batch (Mupd/s)",18}");
+Console.WriteLine(new string('-', 60));
+
+foreach (var (name, stream, batch) in indicators)
+{
+    string streamMups = Mups(TimeNs(stream)).ToString("F1");
+    string batchMups = batch is null ? "-" : Mups(TimeNs(batch)).ToString("F1");
+    Console.WriteLine($"{name,-22}{streamMups,20}{batchMups,18}");
+}
+
+Console.WriteLine(
+    "\nMupd/s = million indicator updates per second. Streaming is the per-tick\n" +
+    "Update path crossing the managed<->C-ABI boundary once per value; batch is\n" +
+    "the bulk array path (one boundary crossing). Higher is better. Numbers are\n" +
+    "machine-dependent - use them for relative comparison, not as a speed claim.");