epic-12: after epic audit and init commit

docs/decisions/adr-0006-event-store-append-only.md

@@ -0,0 +1,76 @@
+---
+name: "Adr 0006 Event Store Append Only"
+description: "Append‑only, ordered, idempotent event store invariants"
+depth: 2
+links: ["../index.md", "../modules/core-events-submodule-spec.md", "./adr-0005-persistence-choice.md"]
+---
+
+# ADR-0006: Event Store Append-Only & Ordering Invariant
+
+**status:** accepted
+**date:** 08.05.2026
+**deciders:** Kami
+
+---
+
+## context
+
+The system relies on an append-only event log as the single source of truth. Replay correctness, session reconstruction, and deterministic projections all depend on strict ordering and immutability of persisted events.
+
+Without enforced guarantees at the storage layer, higher-level invariants (projection determinism, replay correctness) cannot be trusted.
+
+This ADR formalizes storage-level constraints for all EventStore implementations.
+
+---
+
+## content
+
+The EventStore MUST enforce:
+
+### 1. append-only semantics
+
+* events cannot be updated or deleted
+* event_id is immutable and unique per event
+
+### 2. idempotency
+
+* repeated append attempts with same event_id MUST be ignored or rejected safely
+* duplicate events MUST NOT be stored
+
+### 3. per-session ordering
+
+* sequence MUST be strictly monotonically increasing per session_id
+* ordering MUST be total within a session stream
+
+### 4. causality preservation
+
+* causation_id and correlation_id MUST be preserved as metadata only
+* they MUST NOT affect ordering
+
+### 5. deterministic read model
+
+* read(session_id) MUST always return events in sequence order
+* read consistency MUST not depend on implementation timing
+
+---
+
+## consequences
+
+**positive:**
+
+* guarantees replay correctness foundation
+* enables deterministic projection logic
+* simplifies concurrency model (append-only reduces state conflicts)
+* allows multiple backend implementations (sqlite, in-memory)
+
+**negative:**
+
+* no event mutation allowed (requires compensating events instead)
+* stricter implementation requirements for persistence engines
+* potential write amplification in correction scenarios
+
+---
+
+## status
+
+accepted