feat(sse): detect sequence gaps and stale events

[luoye520ww]

Problem

The sequenced SSE contract can identify order, but the consumer still needs a deterministic decision before projecting an event. Applying an event after a missing sequence can corrupt the renderer projection and make reconnect recovery non-deterministic.

Scope

This PR adds a pure sequence detector on top of #902. It does not change the SSE transport, renderer reducer, replay route, or cursor persistence.

Changes

• Classify events as accepted, duplicate, out-of-order, gap, or old-stream.
• Return the expected sequence and a new cursor only for contiguous events.
• Keep the previous cursor for duplicates, stale events, gaps, and stream changes.
• Validate event metadata with the shared sequenced event schema before classification.

Safety

• Gap detection is fail-closed for projection: callers can stop and perform bounded replay or authoritative reload.
• No input event mutates the caller-owned cursor.
• The detector does not claim cross-thread isolation because the current event contract has no thread field.

Typecheck

npm.cmd run typecheck

Passed.

Tests

npm.cmd exec vitest run src/shared/sse-gap.test.ts
npm.cmd run lint
npm.cmd run build
git diff --check

Passed: 6 sequence decision tests; root typecheck, lint, and build passed; diff check passed.

Review performed

• Sequence boundary correctness
• Duplicate and stale event behavior
• Stream replacement behavior
• Cursor immutability
• Contract validation and PR scope

Non-goals

• SSE transport changes
• Renderer reducer integration
• Bounded replay or authoritative reload
• Persistent resume cursors

Dependency

Depends on #902 for SequencedRuntimeEvent. After #902 merges, rebase this branch onto develop and drop dependency commits.

[luoye520ww]

Superseded by consolidated IPC/SSE reliability PR #945, which passed Typecheck, Linux, macOS, and Windows packaged checks. Closing this duplicate to keep the review surface focused.