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.

CHANGELOG.md

@@ -6,6 +6,12 @@ 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]
+### Fixed
+- The R binding's golden-fixture parity test now skips gracefully when the shared
+  `testdata/golden` fixtures are not bundled with the package — standalone
+  r-universe / CRAN builds package only `bindings/r`, so the repo-root fixtures
+  are unreachable there. The parity stays enforced by the repository CI, where
+  the fixtures are present.
 
 ## [0.8.4] - 2026-06-11
 ### Fixed

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

bindings/r/tests/testthat/test-golden.R

@@ -3,23 +3,36 @@
 # archetype test only checks finiteness, this pins exact values, catching wiring
 # bugs (swapped params, wrong multi-output field). Fixtures are generated by
 # `cargo run -p wickra-examples --bin gen_golden`.
+#
+# The fixtures live at the repository root (testdata/golden) and are present
+# during the monorepo test run, but they are NOT bundled into the standalone R
+# package. A packaged check (r-universe / CRAN) therefore cannot find them, so
+# these tests skip there; the parity is already enforced by the repository CI.
 
-golden_dir <- local({
+find_golden_dir <- function() {
   d <- normalizePath(getwd(), winslash = "/", mustWork = FALSE)
   repeat {
     g <- file.path(d, "testdata", "golden")
     if (dir.exists(g)) return(g)
     parent <- dirname(d)
-    if (identical(parent, d)) stop("testdata/golden not found from ", getwd())
+    if (identical(parent, d)) return(NULL)
     d <- parent
   }
-})
+}
+
+golden_dir <- find_golden_dir()
+
+skip_if_no_golden <- function() {
+  skip_if(is.null(golden_dir), "golden fixtures not bundled with the package")
+}
 
 read_golden <- function(name) {
   read.csv(file.path(golden_dir, paste0(name, ".csv")),
            colClasses = "character", check.names = FALSE)
 }
 
+read_golden_input <- function() read.csv(file.path(golden_dir, "input.csv"))
+
 gcell <- function(s) if (identical(s, "nan")) NA_real_ else as.numeric(s)
 
 expect_close <- function(got, want, row, field) {
@@ -31,9 +44,9 @@ expect_close <- function(got, want, row, field) {
   }
 }
 
-golden_input <- read.csv(file.path(golden_dir, "input.csv"))
-
 test_that("scalar indicators match golden", {
+  skip_if_no_golden()
+  golden_input <- read_golden_input()
   specs <- list(c("sma", 14), c("ema", 14), c("rsi", 14))
   for (spec in specs) {
     name <- spec[[1]]
@@ -47,6 +60,8 @@ test_that("scalar indicators match golden", {
 })
 
 test_that("candle Atr matches golden", {
+  skip_if_no_golden()
+  golden_input <- read_golden_input()
   atr <- Atr(14)
   exp <- read_golden("atr")
   for (i in seq_len(nrow(golden_input))) {
@@ -57,6 +72,8 @@ test_that("candle Atr matches golden", {
 })
 
 test_that("pairwise Beta matches golden", {
+  skip_if_no_golden()
+  golden_input <- read_golden_input()
   beta <- Beta(20)
   exp <- read_golden("beta")
   for (i in seq_len(nrow(golden_input))) {
@@ -67,6 +84,8 @@ test_that("pairwise Beta matches golden", {
 })
 
 test_that("multi-output MACD matches golden", {
+  skip_if_no_golden()
+  golden_input <- read_golden_input()
   macd <- MacdIndicator(12, 26, 9)
   exp <- read_golden("macd")
   for (i in seq_len(nrow(golden_input))) {
@@ -82,6 +101,8 @@ test_that("multi-output MACD matches golden", {
 })
 
 test_that("multi-output ADX matches golden", {
+  skip_if_no_golden()
+  golden_input <- read_golden_input()
   adx <- Adx(14)
   exp <- read_golden("adx")
   for (i in seq_len(nrow(golden_input))) {