fix(r): skip golden-fixture test in standalone package builds + document parity (#257)

## Problem
The r-universe build of `wickra` 0.8.4 fails (`R CMD check` ERROR): the golden-fixture parity test added in #255 walks up from the working directory looking for `testdata/golden` and `stop()`s when it cannot find it. Standalone package builds (r-universe / CRAN) package only `bindings/r`, so the repo-root `testdata/golden` fixtures are unreachable there — whereas the monorepo CI checks out the full repo, so the walk-up succeeds and the test passes.

Run: https://github.com/r-universe/wickra-lib/actions/runs/27354800361

## Fix
- **`bindings/r/tests/testthat/test-golden.R`** — the fixture-directory lookup now returns `NULL` instead of erroring when the fixtures are absent, and each test starts with `skip_if(is.null(golden_dir), ...)`. The repository CI (full repo present) still runs the parity checks; standalone builds skip them. The per-test `golden_input` read moved inside the (post-skip) test bodies so nothing runs at source time when the fixtures are missing.
- **`CHANGELOG.md`** — `[Unreleased]` Fixed entry.

## Docs (B6)
- **`README.md` `## Testing`** — the four C-ABI bindings (C#, Go, Java, R) were described as covering one indicator per FFI archetype; document that they additionally replay the shared golden fixture and assert exact parity with the Rust reference outputs.

README.md

@@ -358,6 +358,12 @@ Every layer is covered; run the suites with the commands in
 - `bindings/java`: JUnit cases covering one indicator per FFI archetype
   (scalar/batch, multi-output, bars, profile, array input) plus batch equivalence.
 
+The four C-ABI bindings (C#, Go, Java, R) additionally replay a shared,
+language-neutral golden fixture (`testdata/golden/*.csv`, generated by
+`cargo run -p wickra-examples --bin gen_golden`) and assert exact parity with the
+Rust reference outputs across every archetype (SMA, EMA, RSI, ATR, MACD, ADX,
+Beta), catching FFI wiring bugs the math-only core tests cannot see.
+
 ## Contributing
 
 Contributions are very welcome — issues, bug reports, ideas, and pull requests