fix(build): judge each index by its own source fallbacks (#193)

## Why

#192 made the per-minor objects visible to their pass, and that part works: every slot now reports a populated per-minor cache where it previously reported `0 of 119053`.

But the cache-driven skip rate stayed near **zero**, and the builds kept recompiling packages that already exist. Sampling what pipeline 12027 called missing, against the live CDN:

```
orthoDr_0.6.7.tar.gz      4.4/=404  flat=404   <- genuinely missing
osmdata_0.2.4.tar.gz      4.4/=200  flat=404   <- already built, rebuilt anyway
osmdata_0.2.5.tar.gz      4.4/=200  flat=404
osmdata_0.3.0.tar.gz      4.4/=200  flat=404
osqp_0.6.3.3.tar.gz       4.4/=200  flat=404
outbreaker2_1.1.0.tar.gz  4.4/=200  flat=404
```

`source_served` is read from the **flat** slot's `PACKAGES.gz` and lists the packages that index serves as CRAN source instead of a binary. It was applied to every path in the listing:

```r
existence_cache <- relative_paths[!basename(relative_paths) %in% source_basenames]
```

A package the flat slot serves as source still has a real `4.4/<pkg>.tar.gz`, so dropping that basename deleted a genuine binary from the cache.

The overlap with the work list is total rather than partial, which is why the skip rate was ~0% and not merely low: the sensitive candidate list is exactly "sensitive packages with no flat binary", which is the same set this was removing. amd64/resolute listed 21212 per-minor objects and cached only 13572; the missing ~7600 are these.

## What changed

- Read each per-minor `PACKAGES.gz` and exclude only that minor's own source fallbacks.
- Keep judging flat-slot objects by the flat index.
- Log the fallback count per index, so a future collapse is visible rather than silent.

All per-minor indexes were confirmed reachable (`4.4`, `4.5`, `4.6` return 200 on the slots checked); an unreadable index falls back to keeping that scope's cache in full, which is the safe direction.

## Verification

```
4.4 pass sees -> OLD: osqp_0.6.3.3          NEW: osmdata_0.2.4, osqp_0.6.3.3
all assertions PASS
```

Correct in both directions: the old code dropped `4.4/osmdata` (a real binary) *and* kept `4.6/broken` (a real source fallback in the 4.6 index). The new code restores the first and still excludes the second.

`prek run -a` passes.

Pipelines 12025-12029 were stopped rather than left recompiling what they already have; their uploads persist, so a fresh run inherits them.

Reviewed-on: #193

local/packages-to-build.R

@@ -154,9 +154,10 @@ index_url <- sprintf(
   arch,
   codename
 )
-source_served <- tryCatch(
+read_source_served <- function(index) {
+  tryCatch(
     {
-    con_idx <- gzcon(url(index_url, open = "rb"))
+      con_idx <- gzcon(url(index, open = "rb"))
       on.exit(close(con_idx), add = TRUE)
       idx <- read.dcf(con_idx, fields = c("Package", "Version", "Built"))
       sprintf(
@@ -167,13 +168,16 @@ source_served <- tryCatch(
     },
     error = function(e) {
       cat(sprintf(
-      "WARNING: could not read %s (%s); keeping the full S3 cache\n",
+        "WARNING: could not read %s (%s); keeping that index's cache in full\n",
-      index_url,
+        index,
         conditionMessage(e)
       ))
       character(0)
     }
   )
+}
+
+source_served <- read_source_served(index_url)
 
 binary_cache <- setdiff(file_names, source_served)
 
@@ -225,9 +229,65 @@ if (sum(stripped_per_minor) != sum(per_minor_object)) {
   ))
 }
 source_basenames <- source_served
-existence_cache <- relative_paths[
+
-  !basename(relative_paths) %in% source_basenames
+# A source-fallback list describes ONE index, so it may only be applied to that
-]
+# index's objects. Applied to every path it deletes real per-minor binaries from
+# the cache: a package the flat slot serves as CRAN source still has a genuine
+# `4.4/<pkg>.tar.gz`, and dropping that name makes every per-minor pass rebuild
+# it, every run, forever.
+#
+# The overlap is total rather than partial, which is why this pinned the skip
+# rate near 0%: the sensitive candidate list is exactly "sensitive packages with
+# no flat binary", which is the same set this was removing. amd64/resolute
+# listed 21212 per-minor objects but cached only 13572, then recompiled osmdata,
+# osqp and outbreaker2 while their 4.4 binaries sat in the bucket.
+#
+# So judge each minor by its own index, and the flat slot by the flat index.
+is_per_minor_path <- grepl("^[0-9]+\\.[0-9]+/", relative_paths)
+path_minor <- ifelse(
+  is_per_minor_path,
+  sub("^([0-9]+\\.[0-9]+)/.*$", "\\1", relative_paths),
+  ""
+)
+minors_present <- sort(unique(path_minor[is_per_minor_path]))
+per_minor_source <- lapply(minors_present, function(m) {
+  read_source_served(sprintf(
+    "https://cran.rpkgs.com/%s/%s/latest/src/contrib/%s/PACKAGES.gz",
+    arch,
+    codename,
+    m
+  ))
+})
+names(per_minor_source) <- minors_present
+cat(sprintf(
+  "Source fallbacks per index: flat=%d%s\n",
+  length(source_basenames),
+  if (length(minors_present)) {
+    paste0(
+      ", ",
+      paste(
+        sprintf("%s=%d", minors_present, lengths(per_minor_source)),
+        collapse = ", "
+      )
+    )
+  } else {
+    ""
+  }
+))
+
+served_as_source <- vapply(
+  seq_along(relative_paths),
+  function(i) {
+    scope <- if (nzchar(path_minor[i])) {
+      per_minor_source[[path_minor[i]]]
+    } else {
+      source_basenames
+    }
+    basename(relative_paths[i]) %in% scope
+  },
+  logical(1L)
+)
+existence_cache <- relative_paths[!served_as_source]
 cat(sprintf(
   "S3 cache: %d objects, %d served as CRAN source, %d usable binaries\n",
   length(file_names),