Optimize SPFresh base-delta HBC maintenance

[ajroetker]

Summary

• add base-delta posting maintenance, bounded repair/debt reporting, thresholded folding, and overlay-cache instrumentation for SPFresh/HBC
• add scalable centroid-directory comparison paths and optimized-gate benchmark scripts
• wire DB/runtime maintenance scheduling, resource/elapsed budget checks, crash/recovery proof coverage, and public dense format/version config
• harden dense catch-up posting maintenance so post-finalize maintenance is best-effort and still contributes to global maintenance stats with idle/catch-up source counters

Evidence

• production proof PASS: zig build spfresh-production-test --summary all
• optimized gate rechecked after the warm/cold summarizer fix against /private/tmp/antfly-spfresh-hbc-final-gate-evidence.jsonl; the default gate now FAILS only the read recall denominator checks: read_packed_hbc_hbc, read_flat_rabitq, and read_two_level_rabitq report recall_total=315, while the current gate requires >=1000
• overlay cache admission-skip checks pass under the fixed warm-preferred summarizer (0 skips), so the original cold/warm concern is resolved; rerun read evidence with a >=1000 recall denominator before claiming a full optimized-gate PASS again
• 1M VDBB-shaped write evidence still covers hot/random/semantic/append/mixed overwrite distributions; append-heavy deferred debt remains bounded at dirty/delta/overfull 63, max over-capacity 2

Validation

• zig build lib-db-test --summary all (378/382 passed, 4 skipped)
• zig build lib-data-runtime-test --summary all
• zig build spfresh-production-test --summary all
• bash -n zig/scripts/run_spfresh_hbc_comparison.sh
• python3 -m py_compile zig/scripts/summarize_spfresh_hbc_comparison.py
• git diff --check
• python3 zig/scripts/summarize_spfresh_hbc_comparison.py /private/tmp/antfly-spfresh-hbc-final-gate-evidence.jsonl --optimized-gate check --gate-production-proof-file zig/bench/results/spfresh-production-test.log (expected FAIL on the three stale read recall-denominator checks above)