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)
