fix(build): strip the s3 scheme so per-minor objects reach their pass

`s3_dir_ls()` returns keys with the `s3://` scheme attached, but the contrib
prefix was removed as a fixed substring. That took the prefix out of the middle
of the key and left `s3://4.4/curl_1.0.tar.gz`, so the leading scheme defeated
the `^<minor>/` test in `select_cache_for_pass()`.

Every one of the 21212 per-minor objects on amd64/resolute was therefore read as
a flat-slot object: the primary pass reported "119053 of 119053 objects apply to
this pass" and each sensitive pass got an empty cache and recompiled all 10248
sensitive packages it already had.

This change will:

- anchor the contrib prefix and swallow an optional `s3://` with it
- abort when the strip leaves fewer per-minor paths than the listing held, since
  the failure is otherwise silent and only shows up as a full rebuild
- mark the cache format with a `slot_relative` attribute and read that in
  `build-all.R`, rather than sniffing for a `/`, which misreads a slot-relative
  cache holding no per-minor or Archive object as a legacy one

local/packages-to-build.R

@@ -190,14 +190,44 @@ binary_cache <- setdiff(file_names, source_served)
 # So the cache keeps the path relative to the slot, and `build-all.R` selects
 # the part that matches the pass it is running: the flat slot for the primary,
 # `<minor>/` for a per-minor pass.
+#
+# `s3_dir_ls()` returns keys with the `s3://` scheme attached, so stripping the
+# prefix as a fixed substring takes it out of the middle and leaves
+# `s3://4.4/curl_1.0.tar.gz`. That leading scheme defeats the `^<minor>/` test
+# downstream, so every per-minor object is read as a flat-slot object: the
+# sensitive passes see an empty cache and recompile everything they already
+# have. Anchor the pattern and swallow the scheme with it.
 contrib_prefix <- sprintf(
-  "devxy-rpkgs-binaries/%s/%s/latest/src/contrib/",
+  "^(s3://)?devxy-rpkgs-binaries/%s/%s/latest/src/contrib/",
   arch,
   codename
 )
-relative_paths <- sub(contrib_prefix, "", s3_pkgs, fixed = TRUE)
+relative_paths <- sub(contrib_prefix, "", s3_pkgs)
+
+# The strip is load-bearing and fails silently, so assert it. Both counts are
+# derived from the same listing and use the same shape of pattern, so they must
+# agree exactly; a mismatch means the prefix no longer describes the keys.
+stripped_per_minor <- grepl("^[0-9]+\\.[0-9]+/[^/]+$", relative_paths)
+if (sum(stripped_per_minor) != sum(per_minor_object)) {
+  stop(sprintf(
+    paste0(
+      "S3 prefix strip failed: %d per-minor objects in the listing, %d after ",
+      "stripping /%s/. Example key: %s"
+    ),
+    sum(per_minor_object),
+    sum(stripped_per_minor),
+    contrib_prefix,
+    if (any(per_minor_object)) {
+      s3_pkgs[which(per_minor_object)[1L]]
+    } else {
+      "<none>"
+    }
+  ))
+}
 source_basenames <- source_served
-existence_cache <- relative_paths[!basename(relative_paths) %in% source_basenames]
+existence_cache <- relative_paths[
+  !basename(relative_paths) %in% source_basenames
+]
 cat(sprintf(
   "S3 cache: %d objects, %d served as CRAN source, %d usable binaries\n",
   length(file_names),
@@ -208,6 +238,12 @@ cat(sprintf(
 # Save the S3 file listing for the build step to use as s3_package_cache.
 # This avoids loading s3fs/reticulate in the build container, saving memory for
 # the dependency-installer subprocesses
+# Mark the format explicitly. `build-all.R` has to tell a slot-relative cache
+# from a pre-#191 basename one, and sniffing for a "/" cannot: a new-format
+# cache for a slot with no per-minor or Archive objects holds bare names too,
+# and would be read as legacy and used unfiltered, which makes a per-minor pass
+# believe the flat slot's binaries are its own and build nothing.
+attr(existence_cache, "slot_relative") <- TRUE
 saveRDS(existence_cache, "/mnt/cache/packages/s3_cache.rds")
 # Built from the filtered listing, not the raw one: `s3_dt` is subtracted from
 # the build list below, so a source fallback left in here would exclude the very