### Task A8: End-to-end patch test (guarded) and version bump

**Files:**
- Modify: `tests/testthat/test-patches.R`
- Modify: `DESCRIPTION` (version), `NEWS.md`

**Interfaces:**
- Produces: a guarded e2e test proving a dependent package builds when its failing dependency is patched.

- [ ] **Step 1: Add the guarded e2e test**

```r
test_that("a patched dependency unblocks a dependent build (e2e)", {
  skip_if_not(nzchar(Sys.getenv("BINCRAFT_PATCH_E2E")))
  skip_if_offline()

  patches_dir <- withr::local_tempdir()
  jsonlite::write_json(
    list(list(
      package = "RcppParallel", versions = "*", platforms = list("*"),
      env = list(RCPP_PARALLEL_USE_TBB = "0"),
      reason = "bundled TBB fails on this toolchain"
    )),
    file.path(patches_dir, "registry.json"), auto_unbox = TRUE
  )

  out <- withr::local_tempdir()
  result <- build_binary_package(
    "rts2", tag = "latest", local_output_dir_root = out,
    upload = FALSE, archive = FALSE, patches = patches_dir
  )
  expect_true(isTRUE(result) || identical(result, "skipped"))
})
```

- [ ] **Step 2: Run the e2e test in a container**

Run:

```bash
docker run --rm -e BINCRAFT_PATCH_E2E=1 -v "$PWD":/work -w /work \
  reg.devxy.io/rpkgs/build-env-ubuntu:2604 \
  Rscript -e 'devtools::load_all("."); testthat::test_file("tests/testthat/test-patches.R")'
```

Expected: the e2e test runs (not skipped) and PASSES; build log shows `Applying patch to RcppParallel`.

- [ ] **Step 3: Bump version and changelog**

In `DESCRIPTION`, bump `Version:` to `4.3.0.9999`. Prepend to `NEWS.md`:

```markdown
# bincraft 4.3.0

* `build_binary_package()` gains a `patches` argument: a registry of
  per-package env / configure / Makevars overrides and source diffs that are
  pre-built into patched binaries and served to pak, fixing compiler- and
  OS-specific failures (e.g. RcppParallel) including for transitive deps.
```

- [ ] **Step 4: Commit**

```bash
git add tests/testthat/test-patches.R DESCRIPTION NEWS.md
git commit -m "test(patches): guarded end-to-end test; bump to 4.3.0.9999"
```

---