Fix cross-session chain break attribution

[404SecNotFound]

A modified record in one session could inherit coverage from a later signature when the shared collector stream crossed into another session. The earlier session was then reported as verified_complete even though the next record exposed a broken chain.

This change attributes an ambiguous broken boundary to both adjacent sessions and records it once per session. Both sessions become inadmissible, and sequence_verified rejects the altered position. It adds an end-to-end regression for the sparse signed, multiplexed-stream case and refreshes generated repository counts.

Validation:

• New regression reproduced the false-verification result before the fix and passes afterward.
• Independent F05 review probe passes.
• All 16 repository verification gates pass locally.