Follow-up to the salvaged #100114 commit. Its two-pass anchor selection
scanned steers first and real user rows second, so a transcript shaped
[user A, tool(steer B), ..., user C] anchored the already-consumed steer B
over the newer real request C — the same replay class the PR set out to
fix. Replace it with one reversed positional scan that picks whichever
intent-bearing row is last (real role=user or steer-bearing role=tool),
and make the compressed-transcript steer check count only role=tool rows
(the only place the runtime delivers a steer), so a summary quoting the
marker cannot masquerade as live intent.
Adds S1/S2/S3 regression tests (steer dropped by compaction, steer
surviving in tail, newer user turn after steer) plus alternation and
use-exactly-once assertions.
Teknium