test(renderer): fuzz chat projection replay invariants

[luoye520ww]

Problem

SSE reconnect and replay can deliver duplicated, out-of-order, late terminal, and cross-thread projection actions. Existing reducer tests covered a few hand-written sequences but did not exercise combinations of those events.

Root cause

There was no deterministic replay/fuzz harness for the pure chat projection reducer, so reducer invariants could regress without a reproducible sequence.

Scope

This PR adds renderer test coverage only. It does not change reducer behavior, SSE transport, or runtime state.

Changes

• Generate deterministic bounded action sequences from three fixed seeds.
• Include duplicate sequence observations, delta replay, approval/input replay, runtime errors, terminal events, and snapshots.
• Assert replaying the same sequence is deterministic and never throws.
• Assert a snapshot for another thread cannot replace the active timeline or sequence.
• Keep the existing reducer regression suite unchanged.

Safety

• No production code or persisted data changes.
• The generator uses fixed seeds and bounded input, so failures are reproducible.
• The cross-thread assertion protects the active thread from foreign replay data.

Typecheck

```text
npm.cmd run typecheck
npm.cmd --prefix kun run typecheck
```

Passed.

Tests

```text
npm.cmd exec vitest run src/renderer/src/store/chat-projection-reducer.test.ts src/renderer/src/store/chat-projection-reducer.fuzz.test.ts
npm.cmd run lint
npm.cmd run build
git diff --check
```

Results: 10 reducer tests passed; lint, build, and diff checks passed.

Review performed

• Reducer determinism
• Duplicate/replay handling
• Cross-thread isolation
• Bounded deterministic generation
• Test validity and scope

Non-goals

This PR does not add property-test dependencies or change the event transport. A future PR can connect sequence-gap recovery to the reducer using the existing SSE contracts.

Part of the SSE/replay reliability work.

[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Superseded by consolidated IPC/SSE reliability PR #945, which passed Typecheck, Linux, macOS, and Windows packaged checks.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.