docs: implementation plans for R-minor-sensitive builds

Split into two sequenced plans: bincraft 4.2.0 enhancements (per-minor index +
classifier-driven process_cran_updates) and the pipeline integration (full +
iterative). Spec revised to classify at the orchestration layer instead of a
build_binary_package "auto" mode.

docs/superpowers/specs/2026-06-13-r-minor-sensitive-builds-design.md

@@ -103,22 +103,29 @@ Driven by the bincraft enhancement below; the single build step becomes:
 
 ## Required bincraft enhancements (separate PR, coordinated release)
 
-1. `build_binary_package()`:
-   - Accept `is_r_minor_sensitive = "auto"` — classify the source it already clones via
-     `abi_classify()` and route the artifact to the per-minor slot iff `risky`.
-   - Add `r_minor_sensitive_only` — when `TRUE`, skip (return `"skipped"`) non-risky
-     packages early, after classification, before building.
-2. `process_cran_updates()`:
+`build_binary_package()` needs **no change** — it already accepts a concrete
+`is_r_minor_sensitive` logical and routes the slot accordingly. Classification stays at
+the orchestration layer (mirroring the full-build precompute), which keeps
+`build_binary_package`'s pre-build S3 skip-check and source clone untouched.
+
+1. `process_cran_updates()`:
    - Add `r_minor_detection = c("none", "issue", "classifier")` (default `"none"` to
      preserve current behavior; `"issue"` is today's `filter_r_minor_sensitive` path).
-   - Add `r_minor_sensitive_only`, threaded down to `build_binary_package()`.
-   - With `"classifier"`, pass `is_r_minor_sensitive = "auto"` per package instead of a
-     single run-wide bool.
-3. `upload_package_index()`:
-   - Add per-minor slot support: write/upload `PACKAGES*` (and `Meta/archive.rds`) under
-     `…/contrib/<major.minor>/`, mirroring the generic-slot logic. Invocable per minor.
+   - Add `r_minor_sensitive_only` (default `FALSE`).
+   - With `"classifier"`: for each candidate `(name, version)`, clone the source to a temp
+     dir and call `bincraft::needs_per_minor_recompile()`; pass the resulting concrete
+     logical as `is_r_minor_sensitive` to `build_binary_package()`. When
+     `r_minor_sensitive_only = TRUE`, drop non-risky candidates before building.
+   - Implemented via a small internal helper `classify_r_minor_sensitive(name, tag,
+     source_org_url, local_clone_dir)` returning a logical — the unit-testable seam.
+2. `upload_package_index()`:
+   - Add an `r_minor = NULL` argument. When non-NULL (e.g. `"4.4"`), point the remote dir
+     at `…/contrib/<r_minor>/` and write/upload `PACKAGES*` (and `Meta/archive.rds`) there,
+     mirroring the generic-slot logic. Extract the remote-dir construction into a pure
+     helper `package_index_remote_dir(s3_bucket, arch, codename, r_minor = NULL)` — the
+     unit-testable seam.
 
-These three are the only bincraft changes; detection itself (PR #49) is already merged.
+These two are the only bincraft changes; detection itself (PR #49) is already merged.
 
 ## Scope and cleanup