## Why
A dry run of the prune on `ubuntu-2404-amd64` (pipeline 12494):
```
Dry run: 3013 object(s) left in place. Re-run with --apply to delete.
```
3013 objects on one slot. The recorded verdicts say roughly **230** of that slot's objects actually fail `dyn.load()` anywhere.
The gap is the selector: the prune used `abi_cache_risky_packages()`, the source classifier, which calls **93%** of compiled packages risky where the binaries' own symbols say **4.3%**. Applying it would have deleted ~2800 working binaries and turned each into a source fallback for every client that cannot be routed by R minor.
This is the same mistake #129 fixed in the index writer. The prune was written before the per-object verdicts existed (#200, before #128/#129) and was never brought forward.
## What changed
- Selection comes from `abi_flat_safety`, every `NOT safe` row for the slot regardless of which minor it fails under, since the flat object is served to clients on all of them.
- Matching is by **object**, `<package>_<version>`, not by package name. A package can have a safe flat binary at one version and an unsafe one at another; the old code deleted both.
- The explicit `[pkg ...]` override is unchanged in behaviour and still matches by package name, which is what you want when naming a package by hand.
- The refusal guard moves with the selector. An empty result now means the slot's flat-safety backfill never ran — the exact state rhel8 was found in today — and pruning nothing while reporting success would have hidden it.
## Not changed
Still a dry run unless `PRUNE_APPLY=yes`, and still refuses to delete an object whose package CRAN no longer ships, since there would be no source to fall back to.
## Ordering note
A prune converts each deleted binary into a source fallback. bincraft #131 fixes source fallbacks of *archived* versions being advertised as binaries, which breaks `uvr` outright. That should land before any large prune, or this will manufacture new instances of it.
Reviewed-on: #211
pat-s
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