row 264a (#1769, T1 item 7): bus schema 3k to 3l, sessions below a thread

sessions and session_defaults are new and append-only. thread_claims, message_threads and
thread_opens are rebuilt in place with a session column, and thread_claims gains a generation
counted from claim rows. Each existing thread gets a default session with the thread's id; new
threads get a random one. Holder, per-run, release and delivery triggers key on the session,
and two id triggers keep session and thread ids apart (Duckworth checks 1-7, duckworth-0106).

The broker binds a thread run to a session (the default when the record names none), and
thread.open, thread.send and thread.messages take an optional session. New verbs session.list
and session.create (idempotent on op). Tests: sessions.test.mjs for T1 bus half, T2 and T7 with
a pinned 3k fixture; count, version and verb pins; hand-made test threads write the legacy
default session.

Built on row 236 (8f51900b5), which takes 3j-3k, so this step is 3k-3l by landing order
(row264 Q2, sage-2412). The op values are legacy-3l and migration-3l. gitea-migration.test reads
its store after both steps. Not run yet: every run waits for Sage's CLEAR.

scripts/stack-release.sh

@@ -417,7 +417,7 @@ plugin_checks() {
 # Row 181: the tables a bus writer fills without an event in the same transaction, in schema
 # order. A restore drops their rows after STAMP as it drops events, so the guard counts them too.
 # BUILD.md for row 181 names each writer; the T46 guard classifies every table in the schema.
-COUNTED="role_claims thread_claims decision_events messages deliveries task_snapshots projects threads thread_opens instance_names usage_records"
+COUNTED="role_claims thread_claims decision_events messages deliveries task_snapshots projects threads sessions session_defaults thread_opens instance_names usage_records"
 
 STORE_JS='
 const {DatabaseSync}=require("node:sqlite");const {pathToFileURL}=require("node:url");