docs(events): correct sequence semantics; test cursors across sequence gaps

ADR-0002 described the bigserial seq as gap-free. PostgreSQL sequences are
not: values consumed by rolled-back inserts are never reused. The code only
ever relied on monotonic order (seq > cursor), so this is a documentation
correction plus a regression test that burns sequence values and checks
paging, cursor resumption from inside a gap and stats.

docs/adr/0002-event-immutability.md

@@ -9,12 +9,14 @@ Provenance and replay require that history never changes. Application discipline
 ## Decision
 `cognitive_events` (and `belief_versions`, `personality_versions`, `audit_log`, `approval_decisions`) get triggers that raise on UPDATE and DELETE. Belief changes, corrections, consolidations and retractions are new events referencing old ones. Retention purges, if ever configured, run through a dedicated, audited maintenance path that disables the trigger inside one transaction and records what was purged.
 
-Events use UUIDv7 ids (time-ordered) plus a `bigserial seq` for gap-free cursors. Durable consumers read by `seq`; the in-memory bus is only a wake-up.
+Events use UUIDv7 ids (time-ordered) plus a `bigserial seq` that gives a strictly increasing, monotonic ordering key. Durable consumers read by `seq`; the in-memory bus is only a wake-up.
+
+The sequence is **not** gap-free and nothing may assume it is: PostgreSQL sequences are non-transactional, so a value consumed by a statement that later rolls back is never reused, and values may also be skipped across crashes or cache pre-allocation. Consumers therefore treat `seq` as an ordering cursor only: "everything after N" is `seq > N`, never `seq = N + 1`. A missing value is not evidence of a missing event; corruption is detected by the append-only triggers and integrity checks, not by counting. `internal/events.TestCursorsToleratePlaceholderGaps` guards this.
 
 ## Consequences
 - History is trustworthy by construction; provenance links never dangle.
 - Storage grows monotonically; retention policy is an explicit operator decision.
-- Cursor-based consumption makes restarts and replay simple.
+- Cursor-based consumption makes restarts and replay simple, and tolerates gaps in `seq` by construction.
 
 ## Alternatives considered
 - Event sourcing with projections rebuilt from events only: elegant but forces all reads through replay; we keep mutable projection tables (beliefs, memories) with immutable version tables beside them.