Third review round follow-through: enqueue_tx stamp source, sweep_expired handle form, with_tx signature, and B-block ambiguity pins

docs/architecture/decisions/015-streams-depth.md

@@ -188,9 +188,16 @@ as already pinned):
 > offsets immutable and never renumbered.*
 
 This is the row the contract suite pins cross-engine stream
-equivalence against: two engines, same publish sequence → same offset
-sequence → same `read_since` output order, for direct and subscriber
-reads alike. It is deliberately *not* stronger than both engines can
+equivalence against: two engines, same publish sequence → same
+*(per-stream)* offset sequence → same `read_since` output order, for
+direct and subscriber reads alike. *(Per-stream stated 2026-10-07,
+third review round follow-through: both engines' offsets are global
+counter values — pg bigserial, SQLite AUTOINCREMENT — so a fresh
+stream's first event does not carry offset 1; equivalence means the
+same publish sequence into one stream produces the same *relative*
+offset sequence per that stream, a property safe under either
+counter scheme; cross-stream offset values are engine-internal and
+never compared.)* It is deliberately *not* stronger than both engines can
 implement by a shared SQL shape (`ORDER BY offset ASC` is already
 both engines' claim path) and not weaker than the mechanism's purpose
 (replay demands monotone positions).