feat(r): getting-started vignette + sample_ohlcv dataset (#262)

Fills the two empty r-universe tabs (**Articles**, **Datasets**) for the R package and gives R users a runnable onboarding path.

## What
- **`vignettes/getting-started.Rmd`** — Articles tab. Walks through batch vs streaming (and that they're equivalent), multi-output MACD, candle ATR, and `reset()`, all over the bundled sample series. Built strictly from the already-proven README quick-start + golden-test API (`Sma`/`Ema`/`Rsi`/`Atr`/`MacdIndicator`, `batch`/`update`/`reset`) — no new indicator logic.
- **`data/sample_ohlcv.rda`** (+ `data-raw/sample_ohlcv.R` generator) — Datasets tab. A deterministic, seeded synthetic daily OHLCV series (250 rows × `date/open/high/low/close/volume`); documented via `R/data.R` + `man/sample_ohlcv.Rd`. `LazyData: true` → available right after `library(wickra)`.
- **`DESCRIPTION`** — `Suggests: knitr, rmarkdown`, `VignetteBuilder: knitr`, `LazyData: true`.
- **`ci.yml`** — the R job now knits the vignette (executes its R chunks, no pandoc needed) so a broken example is caught **in CI** before r-universe / CRAN `R CMD check`. The main job otherwise only `R CMD INSTALL`s.

## Verified locally (R 4.6.0 + Rtools45)
- Package installs with the dev C-ABI override; dataset moves to the lazyload DB.
- Vignette **knits cleanly** — every chunk runs, `batch == streaming` holds, MACD/ATR/RSI produce sensible values.

## Notes
- No version bump — metadata/docs only; rides the next release. Merging triggers an r-universe rebuild → Articles + Datasets populate **and** (now that `support@wickra.org` is verified) the maintainer avatar resolves.
- `data-raw/` is `.Rbuildignore`d (generator, not shipped). The `.rda` is XZ-compressed (~3 KB).

Not merging — for review.

.github/workflows/ci.yml

@@ -926,6 +926,16 @@ jobs:
           R CMD INSTALL bindings/r
           Rscript -e 'library(testthat); library(wickra); test_dir("bindings/r/tests/testthat", stop_on_failure = TRUE)'
 
+      - name: Build the vignette code
+        shell: bash
+        # The getting-started vignette runs at R CMD check time on r-universe /
+        # CRAN (with pandoc); this job only INSTALLs, so execute the vignette's R
+        # chunks here (knit, no pandoc needed) to catch a broken example before it
+        # reaches the published build.
+        run: |
+          Rscript -e 'install.packages("knitr", repos = Sys.getenv("RSPM", unset = "https://cloud.r-project.org"))'
+          Rscript -e 'knitr::knit("bindings/r/vignettes/getting-started.Rmd", output = tempfile(fileext = ".md"), quiet = TRUE); cat("vignette code OK\n")'
+
       - name: Run the offline R examples
         shell: bash
         # No loader-path exports: the installed package is self-contained (bundled