Make the R binding self-contained (fetch the C ABI at install time) (#235)

## Problem
The R package built **only** inside the dev/CI workspace. `src/Makevars` and `configure.win` hard-required `WICKRA_INCLUDE_DIR` / `WICKRA_LIB_DIR` pointing at a pre-built `libwickra` (`configure.win` literally `: "${WICKRA_LIB_DIR:?...}"`), and there was no Unix `configure`. So **r-universe** and any plain `install.packages` / `install_github` failed — R had no working end-user install path.

## Fix
Fetch the prebuilt `wickra-c-<triple>.tar.gz` release asset matching the package version at install time and bundle the library into the package (same outcome as the C# / Java bindings, which bundle per-platform native libs):

- **New POSIX `configure`** (the Unix hook R lacked): detect OS/arch → triple, download + `untar` via **base R** (no curl/wget system dep), stage `wickra.h` + `libwickra.{so,dylib}` into `src/`, generate `src/Makevars` from `Makevars.in` with an rpath (`$ORIGIN` Linux, `@loader_path` + `install_name_tool` macOS) so the bundled lib resolves post-install.
- **`configure.win`**: drop the hard `WICKRA_LIB_DIR` requirement; download the Windows triple when unset, then keep the existing `wickra_abi.dll` rename + `objdump`/`dlltool` import-lib dance (sourcing the dll/header from the asset).
- **`install.libs.R`** also bundles `libwickra.dylib` (macOS); the `*.so`/`*.dll` globs already covered Linux/Windows.
- `WICKRA_INCLUDE_DIR`/`WICKRA_LIB_DIR` stay as an optional **dev override**.
- **CI (3 OS)** keeps building against the locally built C ABI (version-independent — avoids the chicken-egg of downloading the in-flight version) but **no longer exports `LD_LIBRARY_PATH`/`DYLD_LIBRARY_PATH`**, so it now verifies the bundled rpath — the real self-contained path users and r-universe get.

`Makevars` is now generated from `Makevars.in`; `SystemRequirements` + ignore/attributes files updated.

## Validation
- The Windows `objdump`/`dlltool` import-lib dance was smoke-tested locally against the real v0.7.9 asset (2412 `wickra_` exports → import lib built).
- Linux/macOS rpath bundling can't be tested on this Windows host → **the 3-OS `r` CI job is the gate** (now without the loader-path mask).
- Follow-up (separate, after release): set up `wickra-lib/wickra-lib.r-universe.dev` (`packages.json` → `bindings/r`).

Closes the R half of the self-contained-distribution gap (`todo-10`).

.github/workflows/ci.yml

@@ -868,23 +868,24 @@ jobs:
 
       - name: Install and test the R binding
         shell: bash
-        # github.workspace is a backslash path on Windows; configure.win (sh) and
-        # mingw need forward slashes. On Linux/macOS the rpath points at
-        # WICKRA_LIB_DIR; export the loader path too as a belt-and-suspenders.
+        # github.workspace is a backslash path on Windows; configure(.win) (sh) and
+        # mingw need forward slashes. WICKRA_*_DIR makes configure use the locally
+        # built C ABI (dev override) instead of downloading the release asset, so
+        # CI is version-independent. Deliberately do NOT set LD_LIBRARY_PATH /
+        # DYLD_LIBRARY_PATH: the lib is bundled into the package and must resolve
+        # via the rpath ($ORIGIN / @loader_path) baked by configure — that is the
+        # self-contained install path real users (and r-universe) get.
         run: |
           export WICKRA_INCLUDE_DIR="${WICKRA_INCLUDE_DIR//\\//}"
           export WICKRA_LIB_DIR="${WICKRA_LIB_DIR//\\//}"
-          export LD_LIBRARY_PATH="$WICKRA_LIB_DIR:$LD_LIBRARY_PATH"
-          export DYLD_LIBRARY_PATH="$WICKRA_LIB_DIR:$DYLD_LIBRARY_PATH"
           R CMD INSTALL bindings/r
           Rscript -e 'library(testthat); library(wickra); test_dir("bindings/r/tests/testthat", stop_on_failure = TRUE)'
 
       - name: Run the offline R examples
         shell: bash
+        # No loader-path exports: the installed package is self-contained (bundled
+        # lib + rpath), so the examples exercise exactly what end users run.
         run: |
-          export WICKRA_LIB_DIR="${WICKRA_LIB_DIR//\\//}"
-          export LD_LIBRARY_PATH="$WICKRA_LIB_DIR:$LD_LIBRARY_PATH"
-          export DYLD_LIBRARY_PATH="$WICKRA_LIB_DIR:$DYLD_LIBRARY_PATH"
           cd examples/r
           for f in streaming backtest multi_timeframe parallel_assets \
                    strategy_rsi_mean_reversion strategy_macd_adx strategy_bollinger_squeeze; do