fix(log): create log file when arming SIGSEGV handler fd

sigsegv::set_log_fd() runs before init_tracing opens/creates the
writer file. Its previous OpenOptions used append(true) with no
create(true), so on the first-ever session the file did not yet
exist, the open failed silently, and LOG_FD stayed -1. When a
segfault later fired the banner was written to fd 2 only; nvim
discards stderr, and nothing ever hit the log — leaving users
with only the terminal banner and no attachable crash section.

Refs #641.

crates/fff-core/src/log.rs

@@ -34,8 +34,15 @@ mod sigsegv {
 
     static LOG_FD: AtomicI32 = AtomicI32::new(-1);
 
+    // Must `create(true)` — this runs before init_tracing opens/creates the
+    // writer file, so an append-only open on a non-existent path silently
+    // fails, LOG_FD stays -1, and the SIGSEGV banner never reaches the log.
     pub fn set_log_fd(path: &Path) {
-        if let Ok(file) = std::fs::OpenOptions::new().append(true).open(path) {
+        if let Ok(file) = std::fs::OpenOptions::new()
+            .create(true)
+            .append(true)
+            .open(path)
+        {
             let prev = LOG_FD.swap(file.into_raw_fd(), Ordering::Relaxed);
             if prev >= 0 {
                 unsafe { libc::close(prev) };