crash_diag: log silent Windows deaths (0xC000041D) to iris-crash.log

[techomancer]

The native CLI dies on some Windows machines during startup with exit code 0xC000041D (STATUS_FATAL_USER_CALLBACK_EXCEPTION) and no message: the fault happens on the stack of a Win32 callback (winit's wndproc), and 64-bit Windows re-raises any exception that escapes a kernel callback as that fixed status, discarding the original cause. The release profile is panic = "abort", so a Rust panic there also prints nothing useful. This is the shape of issue #94.

src/crash_diag.rs, installed first thing in main():

• panic hook - runs even under panic=abort, before the abort; writes message + thread + location + backtrace.
• Windows vectored exception handler - runs first-chance, before the callback dispatcher swallows the exception; logs the real code, faulting address and a module+offset stack for access violations, illegal/privileged instructions, stack overflow, heap corruption and the fatal-callback status itself.
• Windows SetUnhandledExceptionFilter as a backstop.

Everything is appended to iris-crash.log and echoed to stderr. Dormant until something crashes - no steady-state cost (verified: no polling thread, no allocation, handlers only entered on an actual OS exception, never for Result::Err / None / release overflow).

Self-test: IRIS_CRASH_SELFTEST=panic|thread|segv. Escape hatch if the
VEH ever gets noisy: IRIS_CRASH_DIAG=off (panic hook stays).

rules/gui/windows-silent-exit-0xc000041d.md records the investigation: what 0xC000041D means, how to symbolize the report, and which re-entrancy theories were ruled out (vendored winit 0.30.13 buffers the re-entrant request_inner_size path; not reproducible on a desktop; not fixed upstream through winit 0.31-beta).