Robust-slice follow-up to the low-rank bypass. Two changes to _DiffCache:
1. Store fp32, not fp16. A cache hit is now numerically identical to a
recompute, so results are independent of diff_cache_mode and of what fit
in the cache (they match the 'disabled' path). Removes the documented
~1e-5 cache-vs-disabled score shift.
2. 'auto' mode recomputes past the RAM budget instead of spilling dense
diffs to disk (recompute beats disk I/O — that spill once made Pass-2 98%
disk wait). Explicit 'disk' mode is unchanged for those who want it.
Together these make caching purely a perf/memory policy with zero effect on
results. Algo version 1.11.1 -> 1.11.2 (also covers the earlier low-rank
bypass numerics); invalidates rankings cached under the old fp16 regime.
Registry 1.8.0. Tooltips updated; +2 regression tests (277 pass).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
