In C ABI builds, the Zig std.log default writes to stderr which is
not appropriate for a library. Override std_options.logFn with a
custom sink that dispatches to an embedder-provided callback, or
silently discards when none is registered.
Add GHOSTTY_SYS_OPT_LOG to ghostty_sys_set() following the existing
decode_png pattern. The callback receives the log level as a
GhosttySysLogLevel enum, scope and message as separate byte slices,
giving embedders full control over formatting and routing.
Export ghostty_sys_log_stderr as a built-in convenience callback that
writes to stderr using std.debug.lockStderrWriter for thread-safe
output. Embedders who want the old behavior can install it at startup
with a single ghostty_sys_set call.
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