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/signal.rs

@@ -49,7 +49,7 @@ static SCRATCH: ScratchCell = ScratchCell(UnsafeCell::new(Scratch {
     #[cfg(target_vendor = "apple")]
     native: crate::native::Native::new(),
 }));
-static BUSY: AtomicBool = AtomicBool::new(false);
+pub(crate) static BUSY: AtomicBool = AtomicBool::new(false);
 
 /// A `fmt::Write` into a fixed byte slice; errors when full.
 struct SliceWriter<'a> {
@@ -302,4 +302,11 @@ pub(crate) fn install_handlers() {
             libc::sigaction(sig, &raw const sa, std::ptr::null_mut());
         }
     }
+    // macOS: memory faults are Mach exceptions first; the exception port
+    // handler writes the minidump (see `mach_handler`). The signal handlers
+    // above stay as the path for SIGABRT and as a fallback.
+    #[cfg(target_os = "macos")]
+    if !crate::mach_handler::install() {
+        log::warn!("mach exception port handler unavailable; using signal handlers only");
+    }
 }