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.

CONTRIBUTING.md

@@ -107,7 +107,8 @@ The [README](README.md#architecture) has the layer table. The short version:
    `cargo xtask assets` enforces this.
 4. **Never crash.** People trust a CAD app with their work.
    - Non-test code never uses `unwrap`, `expect`, `panic!` or indexing on input-derived data.
-     There's no `unsafe`, and clippy denies it.
+     There's no `unsafe` outside `apps/solidcraft/src/open_documents.rs` (macOS file-open events),
+     and the compiler denies it.
    - An unsupported case returns an error ("not supported yet: …"); it never panics.
    - Every call into truck goes through `solidcraft_kernel::guard`.
    - Every crash fix comes with a regression test.