fix(macos): lazy-load Windows pointer forwarding

[tocekuma]

What and why

Importing wallpaper.wallpaper_engine_bridge on macOS currently imports the Windows-only pointer wheel hook at module load time. That hook references ctypes.WINFUNCTYPE, which is unavailable on non-Windows platforms, so backend startup and test collection fail before wallpaper functionality is used.

This PR keeps the type-only import behind TYPE_CHECKING and loads the Windows implementation only inside the existing sys.platform == "win32" branch. Windows behavior is unchanged.

Linked Issue for product-semantic or public-contract changes: #47

Change class

• Routine fix, documentation, test, maintenance, or presentation-only UI
• Product-semantic or public-contract change discussed in the linked Issue
• Isolated, default-off experiment

Owning layer: wallpaper/wallpaper_engine_bridge.py

User-visible effect, or none: The wallpaper bridge and backend can be imported on macOS and Linux; Windows wheel forwarding remains available when enabled.

Compatibility or migration impact, or none: No configuration or migration required.

Evidence

Commands and manual journeys run:

• python -m pytest -q tests/test_wallpaper_bridge_portability.py tests/test_local_bridge_security.py tests/test_wallpaper_asset_revision.py tests/test_wallpaper_bridge_observability.py — 16 passed

• python -m ruff check wallpaper/wallpaper_engine_bridge.py tests/test_wallpaper_bridge_portability.py — passed

• git diff --check — passed

• Relevant Python tests pass

• CPU/model-less baseline remains supported

• Electron npm run build passes when Electron code changed

• Dependency audit passes when dependencies changed

• Before/after screenshots are attached for visible UI changes

• Documentation/examples are updated for changed settings or contracts

Final check

• This PR addresses one coherent problem without unrelated cleanup
• It does not add a speculative API, fallback, or compatibility path
• No secrets, local state, model weights, voice material, or restricted assets are included
• Third-party notices and provenance are preserved

[Lucas1479]

审查通过。这个改动把类型提示导入放到 TYPE_CHECKING，并只在现有 sys.platform == "win32" 且确实启动滚轮转发时进行运行时导入，修复位置和所有权都正确，没有改变 Windows 行为。

我在 Windows 上复核了相关套件：15 项通过、1 项非 Windows 回归测试按条件跳过；另外确认普通导入不会提前加载 wallpaper.pointer_wheel_forwarder，Ruff 与 git diff --check 通过。与 #46 的合并分析也没有冲突。

该 PR 可以作为独立的跨平台导入修复；批准本身不代表 macOS 原生 Wallpaper 宿主已经完成。

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English

Approved. This change moves the type-only import behind TYPE_CHECKING and imports the Windows pointer forwarder at runtime only when the existing sys.platform == "win32" branch actually starts wheel forwarding. The fix is located at the correct owner and does not change Windows behavior.

I reran the relevant suites on Windows: 15 tests passed and the one non-Windows regression test was skipped as intended. I also confirmed that a normal module import does not eagerly load wallpaper.pointer_wheel_forwarder; Ruff and git diff --check passed, and the merge analysis with #46 was clean.

This PR can land as an independent portability fix. The approval does not mean that the native macOS Wallpaper host is complete.