fix(electron): guard POSIX-only O_NONBLOCK assertion on Windows

[rqaefaqa]

Summary / 摘要

Fix the remaining Windows-only test failure (O_NONBLOCK assertion) and add regression coverage asserting LinkIndex behaves identically for \-separated paths — the normalization contract #77 relied on.

Motivation / 背景与动机

Related to #77 (same class of Windows-only test assumptions). #77's path-separator half was fixed in bb83c3a, but the full suite on Windows 11 still shows one real failure: constants.O_NONBLOCK is undefined on Windows, so openedFlags & constants.O_NONBLOCK evaluates to +0 and the unconditional assertion fails. Test-only change — OpenSpec not required (CONTRIBUTING §3.1).

Changes / 变更内容

• test/plugin-host-broker.test.ts: guard the O_NONBLOCK assertion with the same !== undefined check the test already uses for O_NOFOLLOW two lines below. Production code already passes (constants.O_NONBLOCK ?? 0) — no production change needed.
• test/link-index.test.ts: new path separator normalization describe — \-separated indexing produces results identical to / input (names/files/resolve/backlinks/unlinked mentions), and incremental ops (updateFile/renameFile/removeFile) plus mixed-separator lookups normalize correctly. Deterministic on any OS.

Testing / 测试

• pnpm test — 900/900 on Windows 11, Node 22.20.0, pnpm 9.15.4 (was 1 failed / 898 total before)
• pnpm typecheck + pnpm build — pass on Windows
• New vitest cases: 2 path separator normalization tests + guarded O_NONBLOCK assertion

Compliance / 合规自检

• AI disclosure: drafting/research AI-assisted; diagnosis verified on real Windows before & after; changes reviewed line-by-line.
• CLA — will sign on the bot's prompt (first-time contributor)
• No new dependencies / build artifacts / secrets

Checklist / 自检清单

• Conventional Commits title; test-only change — no public API, no live-preview-table.ts, no capability/breaking change
• In scope per GOVERNANCE.md §4 (apps/electron-demo)

[CLAassistant]

All committers have signed the CLA.