dash-spv/sml: stranded filter ranges, header-sync wedges, and crafted-message masternode stalls

[bfoss765]

Found by the 2026-08-19 merged-PR audit; robustness gaps in code merged via #964/#950/#960/#934:

• Filter-header hole repair: a storage-write failure permanently strands the range below target — the watermark advances before the fallible work, and tick errors are swallowed.
• Header-sync wedges: a window-skip strand, and a pending-announcement retry loop with no exit while Syncing.
• Masternode QRInfo hardening (both empirically reproduced): unvalidated quorum_index and a cl-sig map keyed from unvalidated wire data — a single crafted message wedges masternode sync (fail-closed but permanent until restart).
• Silent empty-filter read: a missing segment file reads as all-empty in release builds — the debug guards should be release errors.
• Smaller: take-then-fail header drop, zero-peer tick guard.

A complete fix exists on branch fix/sync-robustness-and-masternode (former PR #972, closed to keep the open-PR queue focused on migration-critical work; one CodeRabbit minor open — failed-init atomicity test coverage). Reopen as-is when there is review bandwidth.

Related follow-ups registered during review: sibling requeue holes in the blocks/filters pipelines (same class as the return_unsent fix in this branch), and the receive_headers pre-validation locator advance (separate ticket-worthy).