Add native Windows toast notifications

[zcourts]

Summary

• add native Windows local toast delivery, settings, and cancellation through WinRT
• keep packaged identity authoritative and use an explicit AppUserModelID only as the unpackaged fallback
• expose opt-in desktop configuration plus generic x64/ARM64 shortcut-identity scaffolding for NSIS installers
• make the Windows identity builder a true no-op on non-Windows targets so custom macOS and Linux notification hosts remain untouched

Stack

• based directly on 0857c9fe5e09dc019290a55e7b05a6b6e0d46a44, preserving the complete currently pinned SSR, theme, and initial-window tree
• changes exactly five source/manifest files; Cargo.lock is unchanged
• generated scaffold contains only generic example data

Validation

• git diff --check 0857c9fe..HEAD
• rustfmt --edition 2021 --check on all four changed Rust files
• cargo metadata --no-deps --format-version 1 --locked --offline
• recovered focused Rust validation: 2 command-core scaffold tests and 7 notification tests passed
• recovered desktop shell check passed
• recovered native validation compiled and exercised the shortcut helper with x64 and ARM64 MSVC toolchains, including persisted AUMID verification
• recovered stock NSIS include compile passed

A full rebuild was not repeated after the clean transplant because the shared host root had only 4.4 GiB free; the existing shared Cargo target was preserved and not cleaned.