Harden macOS alert action handling

[HituziANDO]

Problem

The macOS alert stores three action handlers in separate properties and calls fatalError when a fourth action is added. Adding or changing actions requires editing multiple branches.

Scope

• Store button metadata and handlers in an ordered collection.
• Map NSApplication.ModalResponse to the correct collection entry.
• Return a recoverable failure if an explicit action limit remains necessary.
• Preserve shortcut-key behavior.

Acceptance criteria

• Adding an action does not require another dedicated handler property.
• No supported action count terminates the process with fatalError.
• Button order, shortcuts, and callbacks remain correct.

Tests

• Cover zero through several actions, response mapping, missing handlers, and shortcut keys on macOS.

Relevant files

• Framework/Sources/Alert.swift