M4 — Transport + store realign

[nficano]

Goal

Transport + store realign.

Scope

• Salvage _transport/ verbatim (move from transport/ to _transport/; add public re-export from arcp/__init__.py)
• Rewrite src/arcp/_store/eventlog.py schema to add session-scoped event_seq column + read_since_seq + ack-aware GC
• Split idempotency into src/arcp/_store/idempotency.py
• Update src/arcp/_store/schema.sql
• Tests for transport round-trip and event-log replay

Spec §

§4, §6.3, §6.5, §8.3

Risk

M (event_seq atomicity invariant)