test(golden): language-neutral fixtures + per-binding parity runners (#255)

**Task 5 — golden-fixture parity for the C-ABI bindings.** Lifts the C#/Go/Java/R tests from *one indicator per archetype* toward reference-value parity, catching FFI wiring bugs (swapped params, wrong multi-output field) the math-only core tests cannot see.

## What's here
- **`examples/rust/src/bin/gen_golden.rs`** + **`testdata/golden/*.csv`** — a Rust generator computing a deterministic OHLCV series plus the core's reference outputs for a curated archetype-spanning set: scalar (`Sma`/`Ema`/`Rsi`), candle (`Atr`), scalar multi-output (`MACD`), candle multi-output (`ADX`), pairwise (`Beta`). `nan` marks warmup. Regenerate with `cargo run -p wickra-examples --bin gen_golden`.
- **Parity runners** replaying the identical fixtures through each FFI (rel-tol 1e-6), each a standard test in the binding's existing suite (no `ci.yml` change — rides `dotnet test` / `go test` / `mvn install` / `R CMD`+testthat). A walk-up search locates `testdata/golden` regardless of run dir.
  - **C#** (`bindings/csharp/.../GoldenTests.cs`) — ✅ validated locally, 7/7 pass.
  - **Go** (`bindings/go/golden_test.go`) — ✅ validated locally, pass.
  - **Java** (`bindings/java/.../GoldenTests.java`) — modeled on the archetype API; validated by CI (no local mvn).
  - **R** (`bindings/r/tests/testthat/test-golden.R`) — modeled on the archetype API; validated by CI (no local Rscript).

## Notes
- The curated set spans every marshalling archetype; extending the indicator list is mechanical (add to the generator + regenerate). Bars/profile archetypes can be added next.
- No new CI jobs.

testdata/golden/beta.csv

@@ -0,0 +1,81 @@
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+nan
+nan
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