fix: prevent sparse-entropy overlap bypass in collision checks

[createkr]

Summary

Hardens and profile comparison to prevent sparse-entropy payloads from weakening collision decisions.

Changes

• add shared core entropy field list + helper counters
• compare fields only when both stored/current values are non-zero (true comparable overlap)
• require minimum comparable overlap () before collision matching
• treat zero-overlap comparison as low-confidence ()
• add regression tests for sparse-overlap bypass pattern and overlap-quality behavior

Validation

• manual sanity script with temp DB verifies sparse-overlap payload does not trigger weak collision decisions

Fixes #396

[Scottcjn]

Merged. Good security fix - the sparse entropy bypass was a real vulnerability. Note: the original bug was reported by @yw13931835525-cyber in issue #383.

[Scottcjn]

Payment confirmed — 15 RTC sent to wallet RTC1d48...2f35 for sparse-entropy overlap security fix. Good catch on the attack vector.

Batched with PR #394 (5 RTC) and PR #389 (5 RTC) = 25 RTC total. Pending ID: 430, confirms in 24h.