🐛 DB-owned message sequences — fix broken conversation ordering

[Robdel12]

Summary

• DB is the sole authority for sequence assignment. INSERT computes MAX(sequence)+1 — app code never assigns sequences.
• Persist-first broadcasts. Row commands use a oneshot response channel to get the DB-assigned sequence back before broadcasting to clients. Zero divergence between in-memory and persisted state.
• Removed all app-layer sequence computation. msg_counter/fetch_add removed from Claude connector, conditional assignment removed from transition.rs.
• V029 migration fixes historical broken sequences (82/86 rows with sequence=0) using ROW_NUMBER() OVER (PARTITION BY session_id ORDER BY rowid).

Why

Dogfooding revealed Codex direct sessions were missing user messages. Root cause: sequences were computed independently in 4 places (Claude connector, Codex connector, transition.rs, SessionHandle). Any restart, restore, or path disagreement could corrupt ordering permanently. Now there's one code path and one source of truth.

How it works

1. Entry arrives with sequence: 0
2. RowAppend/RowUpsert persist command includes oneshot::Sender<u64>
3. DB computes sequence via COALESCE(MAX(sequence)+1, 0) on INSERT
4. execute_command reads back assigned sequence and sends via oneshot
5. Writer flushes immediately when any command has a response channel
6. Handler awaits response, updates in-memory row, then broadcasts

Key files

File	Change
persistence/mod.rs	DB-computed sequences in SQL, readback via oneshot
persistence/commands.rs	sequence_tx on RowAppend/RowUpsert, removed Clone
persistence/writer.rs	Immediate flush when response channels present
session_command_handler.rs	3-pass persist→await→broadcast in transition path
connector-claude/src/lib.rs	Removed msg_counter, all entries use sequence=0
connector-core/src/transition.rs	Removed conditional sequence assignment
session.rs	Added set_row_sequence(), row_by_id()
session_actor.rs	Mock writer in tests, test outcomes not implementation
V029__fix_message_sequences.sql	Fix historical broken sequences

Test plan

• make rust-ci — fmt + clippy + 437 tests pass
• Run server, open broken session → user messages visible after migration
• New session: send messages, verify SELECT sequence FROM messages increments
• Pagination works, streaming smooth