fix(iolog): repair reset sequences and report the true poll head

Read tail bytes before decoding; scan the locked log for its global maximum
and every non-increasing adjacent sequence. Repair retained rows in file order
above that maximum without reserializing their payloads. Trim by position.

Return the snapshot head even for oversized cursors, without skipping capped
pages. Probe engine-room hosting only after classifying a reserved bind id.

Windows and Linux gates on base 914805d1 passed: workspace bins, xtask check,
traceability, clippy, 24 store units, 40 API units, and seven real CLI cases.
UTF-8, position-retention, cursor, and eager-probe mutations all turned red;
all mutations were restored before the final green gates.

Fixes BigscreenVR/spt-bs-releases#277
Fixes BigscreenVR/spt-bs-releases#279

Co-authored by: todlando