feat(crash): port Ghostty's local crash reporting

Ports ZIG/src/crash/* as crates/ghostty-crash. Like Ghostty (which uses
sentry-native only to write local files), crashes are written as Sentry
envelopes (<uuid>.ghosttycrash) to $XDG_STATE_HOME/ghostty/crash:
- envelope parse/serialize (all Zig tests ported)
- panic hook plus SIGSEGV/SIGBUS/SIGILL/SIGFPE/SIGABRT handlers on an
  alternate stack; the signal path is allocation-free and re-raises with
  the default action so exit status and core dumps are unchanged
- thread-local crash context (main/renderer/io/read thread, surface
  dimensions) set from the corresponding threads
- `sentry` cargo feature on the root binary, off by default like
  upstream's -Dsentry; +crash-report lists exactly what is written

Deviation: attachments always carry "type":"attachment" (Zig's
in-memory attachments omit it and become unparseable).

Test Plan:
- cargo test -p ghostty-crash (19)
- cargo test --test crash (child panics, SIGSEGV, stack overflow and
  abort each produce a valid envelope listed by +crash-report)

crates/ghostty-crash/src/lib.rs

@@ -0,0 +1,24 @@
+//! Local crash handling (port of `ghostty/src/crash`).
+//!
+//! This covers setting up the process to catch crashes, writing crash
+//! 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.
+#![cfg_attr(windows, allow(dead_code))]
+
+pub mod context;
+pub mod dir;
+pub mod envelope;
+pub mod sentry;
+#[cfg(unix)]
+mod signal;
+#[cfg(windows)]
+#[path = "signal_windows.rs"]
+mod signal;
+
+pub use context::{DimsSnapshot, SurfaceDims, ThreadGuard, ThreadType, current_dims};
+pub use dir::{Dir, Report, ReportIterator, default_dir};
+pub use envelope::Envelope;
+pub use sentry::{Options, deinit, init, install};