fix(bdk): harden wallet cache miss tracking

[dolcalmi]

Summary

• refactor the pg wallet cache miss-tracking internals behind shared Tracked, PendingSet, and MissTracker helpers
• unify confirmed-and-pending lock acquisition in MissTracker and document the existing extend_* clear-before-insert window
• add regression and deterministic race tests for miss clearing, pending requeue/drain behavior, invalidate reset, and raw transaction preservation during summary refreshes