ci(cache): prune obsolete Soldr cook-base generations

[zackees]

Summary

• Pin every first-party setup-soldr call to runtime Soldr 0.9.23 and guard that policy against mixed producer versions.
• After all CI producers finish on a successful main push, prune only main-ref cook-base entries from older Soldr versions. Cook bases include the Soldr version in their key and restore exact-only, so old-version cook bases are unreachable from current producers.
• Refuse deletion unless a current-version main cook base exists; treat a concurrent delete 404 as already absent; poll inventory and quota until settled.
• Enforce a 9.5 GiB Actions cache budget using the larger of the usage endpoint and listed cache bytes. Preserve all current-version shapes, other cache families, and non-main refs.

Evidence

• RED: current cache usage/listing is 10,829,310,998 bytes. Four v0.9.22 main cook bases total 7,800,315,246 bytes; read-only dry run projects 3,028,995,752 bytes after pruning.
• GREEN: focused unittest suite passed (47 tests); full ci unittest discovery passed (131 tests); git diff check is clean.
• Read-only clud-review clean after follow-up fixes.

Fixes #355. Part of setup-soldr#528.