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/Cargo.toml

@@ -0,0 +1,57 @@
+[package]
+name = "ghostty-crash"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Local crash capture (Sentry envelope files), port of ghostty/src/crash"
+
+[features]
+# Mirrors upstream's `-Dsentry` build option (default off on Linux): when
+# disabled `init()` is a no-op. `install()` always works (used by tests).
+sentry = []
+
+[dependencies]
+ghostty-os.workspace = true
+log = "0.4.34"
+serde_json = { version = "1", features = ["preserve_order"] }
+thiserror = "2.0.21"
+
+[target.'cfg(unix)'.dependencies]
+libc = "0.2"
+
+[dev-dependencies]
+tempfile = "3.27.0"
+
+# The fatal-signal handler needs sigaction/sigaltstack/raw syscalls. Unsafe is
+# denied crate-wide and only allowed in `signal.rs` with SAFETY comments.
+[lints.rust]
+unsafe_code = "deny"
+
+[lints.clippy]
+pedantic = { level = "warn", priority = -1 }
+todo = "warn"
+unwrap_used = "warn"
+# Porting a low-level terminal emulator involves a lot of intentional
+# numeric casts and long state-machine functions; these pedantic lints are
+# noise for this codebase.
+cast_possible_truncation = "allow"
+cast_possible_wrap = "allow"
+cast_sign_loss = "allow"
+cast_precision_loss = "allow"
+cast_lossless = "allow"
+module_name_repetitions = "allow"
+too_many_lines = "allow"
+missing_errors_doc = "allow"
+missing_panics_doc = "allow"
+similar_names = "allow"
+struct_excessive_bools = "allow"
+fn_params_excessive_bools = "allow"
+match_same_arms = "allow"
+unreadable_literal = "allow"
+many_single_char_names = "allow"
+doc_markdown = "allow"
+must_use_candidate = "allow"
+return_self_not_must_use = "allow"
+used_underscore_binding = "allow"
+large_stack_arrays = "allow"
+inline_always = "allow"