ci: stop caching unused cross-target cook bases

[zackees]

Summary

• Disable setup-soldr cook only on the five ci.yml build-matrix target shapes, based on exact-main full run 36281990983: all five reported 0 cook hits. Keep the independent build-cache, prepared-state, registry (cargo-registry-cache: true explicitly), and toolchain cache layers enabled.
• Extend main-only retention to retire exactly the five no-longer-produced current-version xbuild target cook-base shapes, while requiring and preserving the active native xlinux current-generation cook base. Keep old-version pruning, ref/family boundaries, endpoint/list convergence, and fail-closed budget checks.
• Add RED/GREEN policy and retention tests to the CI guard suite.

Evidence

The five full cook archives written by 36281990983 totaled 11,337,504,296 B (~11.34 decimal GB): Linux ARM 2,384,323,319 B; Windows x64 2,731,930,727 B; Windows ARM 2,670,166,683 B; Apple x64 1,776,738,973 B; Apple ARM 1,774,344,594 B. Their measured cook-reuse summaries all showed 165–170 misses and zero hits. Current list and usage converged at 10,481,228,587 B, including 7,764,511,133 B in three surviving cross-target cook bases. Excluding these and seeding the retained native xlinux sentinel projects ~2.844 GB after the main cleanup; the fully populated no-cook producer cycle is estimated around 8.1 GiB and must be verified after merge.

Validation: 113 related CI unit checks passed, including the new RED/GREEN tests; Ruff check/format, workflow YAML parse, and git diff check passed. Apply ci-full for the platform/target-sensitive workflow change. Linked to kernal-api#355 and parent setup-soldr#528; #355 must remain open until exact-main full-cycle usage and cache-retention behavior are verified.