fix: allow Windows short MSIX update URLs

[Wangnov]

Summary

• allow stable Windows update short URLs like /latest/win during manifest/checksum binding
• keep direct .msix URL artifact mismatch protection
• add regression coverage for the short URL path

Fixes #88

Verification

• rustfmt --check --edition 2021 src-tauri/src/app/win_update.rs
• cargo test --manifest-path src-tauri/Cargo.toml app::win_update::tests -- --nocapture
• cargo clippy --manifest-path src-tauri/Cargo.toml --workspace --all-targets -- -D warnings
• cargo test --manifest-path src-tauri/Cargo.toml --workspace
• codex review --uncommitted

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