crash: a panic writes itself down beside the init file, where stderr cannot lose it

Every crash this program has ever had went to stderr and nowhere else, and
stderr is the one place it cannot keep anything. In the tty shell stderr IS the
screen, so the trace lands on the grid the terminal is being reset out of; the
SDL and AppKit shells have no terminal at all; a --detach session's goes
wherever its launcher left it. src/crash.zig appends a record to
<config dir>/crashes first: one line naming the build (version, commit, UTC,
os-arch, pid) and under it the panic message and the frames behind it.

RETURN ADDRESSES and not the symbolised trace, which is measured rather than
chosen. `std.debug.writeCurrentStackTrace` called from a panic handler BEFORE
defaultPanic wedges the process at 0% CPU: symbolising reads DWARF, that read
can itself panic, and the staging which turns a nested panic into "aborting due
to recursive panic" is defaultPanic's own and private. Reproduced in a
standalone build with this program's std_options_debug_io and inside a test
binary. `captureCurrentStackTrace` only walks frames, so the addresses go in
the file and `addr2line -e` finishes the job; stderr still gets the symbolised
trace from defaultPanic, unchanged.

The AppKit shell gets a panic handler of its own here too: the macOS build
roots at macos.zig, so main.zig's had never run there — in the shell with the
least useful stderr of the four. The config directory is COPIED rather than
borrowed, because that host's lives in an arena its own errdefer frees. One
record at a time, so two panicking threads cannot interleave into one buffer.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_016Q4RATpafkwahrovHQLKRf

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