fix(macos): open files double-clicked in Finder (#1031)

macOS hands files opened from Finder, the Dock or open -a to the application delegate's application:openURLs:, which winit's delegate lacks, so AppKit showed "SolidCraft cannot open files in the SolidCraft design format". open_documents adds that method to the delegate's class before launch finishes and queues the paths for the UI, which opens them like files named on the command line.

This is the one place SolidCraft's own code uses unsafe: the app crate denies unsafe_code instead of forbidding it, and only the two macOS-only functions in open_documents.rs allow it, with the reason documented there and in AGENTS.md. Adapted from VectorCraft (see ATTRIBUTION.md).

Closes #997.

AGENTS.md

@@ -27,7 +27,9 @@ new uses of it anywhere else. Public docs/web/Flatpak deployments use project Pa
 ## Never crash
 People trust SolidCraft with their designs; a crash loses their work. **This outranks feature work.**
 - **No panics in non-test code:** no `unwrap()`, `expect()`, `panic!`, `unreachable!`, `todo!`,
-  `unimplemented!`; no `unsafe` (`unsafe_code = "forbid"`).
+  `unimplemented!`; no `unsafe` (`unsafe_code = "forbid"`). The one exception is two functions in
+  `apps/solidcraft/src/open_documents.rs` (that crate denies it instead): macOS hands Finder files
+  only to an Objective-C method.
 - **Errors are `Result<T, E>`** through the crate's error type and `?`. An unfinished feature
   returns an error ("not supported yet: …"); it never panics.
 - **Input-derived numbers are hostile** (command parameters, design files, control-channel