feat(sse): persist bounded resume cursors

[luoye520ww]

Problem

Renderer SSE acknowledgement state is currently in-memory. A renderer/runtime restart loses the last confirmed sequence and can replay an unnecessarily large thread window.

Root cause

There is no bounded, atomic persistence layer for per-scope resume cursors.

Scope

This PR adds only the shared cursor contract and a main-process persistence service. It is independent from IPC and renderer recovery wiring.

Changes

• Add strict, versioned SseResumeCursor and file envelope schemas.
• Persist at most 256 scopes with atomic writes and serialized mutations.
• Reject same-generation cursor regressions.
• Allow a new runtime generation to reset its sequence.
• Keep malformed files untouched and fail open to authoritative replay.
• Ignore storage entries whose key does not match their cursor scope.

Non-goals

• No preload/IPC bridge changes.
• No renderer subscription changes.
• No automatic cursor restore yet; that is a follow-up integration PR.

Safety

• Cursor data contains only bounded IDs, sequence numbers, generation metadata, and timestamps.
• Writes use the existing atomic file helper.
• Corrupt data is never overwritten automatically.
• Entries are capped to prevent unbounded user-data growth.

Typecheck

npm.cmd run typecheck and npm.cmd --prefix kun run typecheck passed on the clean develop-based branch.

Tests

npm.cmd exec vitest run src/main/services/sse-resume-cursor-store.test.ts ? 1 file, 5 tests passed.

Also passed: git diff --check.

Review performed

• Functional correctness
• Concurrency and lifecycle
• Data integrity and atomic writes
• Corrupt-file and generation-boundary handling
• Cross-platform path handling
• PR scope and diff cleanliness

Issue

Part of #885

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Superseded by consolidated IPC/SSE reliability PR #945, which passed Typecheck, Linux, macOS, and Windows packaged checks.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