Add Windows notification handles

[Peter7896]

Draft for #186, Windows side only.

This keeps the existing Windows show() behavior intact and adds an opt-in show_handle() path for callbacks. The first pass stays on the current Windows backend because it already exposes activation and dismissal handlers, so this avoids a backend swap while still proving the handle surface.

What changed:

• add a Windows NotificationHandle with wait_for_action, wait_for_action_response, and on_close
• map Windows activation into action ids, with default for body activation
• map Windows dismissal reasons into CloseReason
• expose the Windows handle/action/close types on Windows
• make the action and close examples run real Windows paths instead of printing the XDG-only message
• add Windows unit coverage for action mapping, closed compatibility, and close reason preservation

Validation run on Windows:

• cargo check
• cargo check --target x86_64-pc-windows-msvc --examples
• cargo test --target x86_64-pc-windows-msvc --no-run
• cargo test --target x86_64-pc-windows-msvc wait_for_action -- --nocapture

This is intentionally still a draft. I still need to attach the requested clean recording before marking it ready for review. My current Windows shell delivered the handle event, but did not render a clean banner capture, so I am not treating this as ready yet.

Refs #186

[Peter7896]

Added a Windows recording for the action callback path:
https://github.com/Peter7896/notify-rust/blob/peter7896/pr-283-windows-action-recording/evidence/pr-283-windows-action-demo.mp4

The recording shows:

• target\x86_64-pc-windows-msvc\debug\examples\actions.exe waiting for a toast action
• the Windows toast rendering button a and button b
• clicking button a returning clicked a to the caller

Local validation already run:

• cargo fmt --check
• git diff --check
• cargo check
• cargo check --target x86_64-pc-windows-msvc --examples
• cargo test --target x86_64-pc-windows-msvc wait_for_action -- --nocapture