[ifdotpy]

Follow-up of #118. layr status compared our live folder once per merge base (about 15 bases in Studio). Each compare walked the live folder again.

Change:

• Our live folder is compared once, with our head state (one live compare per call).
• Per base, the compare of base and our head is a saved state compare, cached as in perf(status): Cache overlap compares of saved states #118.
• The changes against the base are the combination of the two parts. A path that both parts changed is checked again against the base with the same test as the direct compare.
• A rename, or a file that the parts show as added and removed and that git would pair, falls back to the direct compare.
• Overlay layers use the same method: one live compare per overlay.
• Output unchanged. t97 checks the overlaps against the 0.13.1 output (rows and full JSON).

Tests: t97-status-overlaps-bases (new, in the macOS and overlayfs lists), t93 (cache), plus the unit tests of the merge logic.