feat(crash): Breakpad-style minidumps on macOS and Windows

Ghostty links sentry-native with the breakpad backend on every platform
(SharedDeps.zig), so it writes minidumps on macOS and Windows as well.
- macOS: a Mach exception port handler thread (structured like
  Breakpad's exception_handler) catches EXC_BAD_ACCESS/BAD_INSTRUCTION/
  ARITHMETIC/BREAKPOINT, suspends other threads, reads the crashed
  thread state and writes a minidump via minidump-writer as the
  event.minidump attachment, then restores the previous ports so the
  BSD signal and default termination follow. The signal path remains
  the fallback (and handles SIGABRT); a watchdog alarm bounds dumping.
- Windows: SetUnhandledExceptionFilter hands off to a pre-started dump
  thread using minidump-writer's dump_crash_context.
- The screen-file open action decodes the escaped path to raw OS bytes
  (also for the C action passed to embedders).

macOS/Windows code is compile-checked (clippy -D warnings) only; the
shared Mach message decoding and minidump attachment logic have Linux
unit tests. Platform matrix in docs/rewrite/notes/crash-CR.md.

Test Plan:
- cargo test --workspace (3348 passed)
- just check-macos; just check-macos x86_64-apple-darwin; just check-windows

crates/ghostty-crash/src/lib.rs

@@ -4,8 +4,8 @@
 //! reports (Sentry envelopes) to disk, and introspecting them. Nothing is
 //! ever sent over the network.
 
-// Fatal-signal handling does not exist on Windows (Zig skips crash capture
-// there), so parts of the shared report writer are unused.
+// The Windows exception filter does not use the signal-safe report writer,
+// so parts of it are unused there.
 #![cfg_attr(windows, allow(dead_code))]
 
 pub mod context;
@@ -13,6 +13,9 @@ pub mod context;
 mod darwin;
 pub mod dir;
 pub mod envelope;
+mod exception;
+#[cfg(target_os = "macos")]
+mod mach_handler;
 #[cfg(target_os = "linux")]
 mod minidump;
 // Pure (platform independent) event-context formatting; only used by the