Implements bounded read_range and read_by_correlation variants with paged resume, at-least-one guarantee, and budget fallback to OpenOptions defaults. Fixes CursorInner::Correlation field (after_timestamp_us→last_seen_id) to match the id>last_seen_id resume predicate.
Extracts write_degraded_event into SystemStreamWriter; dispatcher thread holds one dedicated SQLite connection outside the write mutex. Byte-identical SubscriptionDegraded output. Phase 1 anchor comments added for Track 2 merge safety.
Adds ReadOutcome, TruncationCursor, BudgetKind, SamplingMode, and dispatch channel observability as the foundation for bounded reads; no existing API surfaces changed.