Audit writer: open a record-less log, share the log-name rule, keep the lock file

M3a review findings 1, 2, 4 and part of 5. One empty log file made brokerd
panic at startup (files[len - 2]); it now opens as an empty log. brokerd's
name check tested one month digit, so a file bxctl ignored could become
brokerd's latest file; both now use proto::is_audit_log_name. Writer no
longer unlinks audit/.lock, which opened a two-writer window. No unwrap in
short_check.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/bxctl/src/verify.rs

@@ -17,7 +17,7 @@ pub fn run(home: &Path, out: &mut dyn Write) -> std::io::Result<bool> {
             Ok(name) => name,
             Err(_) => continue,
         };
-        if is_audit_log(&name) {
+        if proto::is_audit_log_name(&name) {
             names.push(name);
         }
     }
@@ -65,26 +65,3 @@ pub fn run(home: &Path, out: &mut dyn Write) -> std::io::Result<bool> {
     }
     Ok(true)
 }
-
-/// True if `name` is a `YYYY-MM-DD.jsonl` audit log: ten characters, digits with `-` at positions
-/// 4 and 7, then `.jsonl`.
-fn is_audit_log(name: &str) -> bool {
-    let date = match name.strip_suffix(".jsonl") {
-        Some(date) => date,
-        None => return false,
-    };
-    if date.len() != 10 {
-        return false;
-    }
-    let bytes = date.as_bytes();
-    for (i, &b) in bytes.iter().enumerate() {
-        if i == 4 || i == 7 {
-            if b != b'-' {
-                return false;
-            }
-        } else if !b.is_ascii_digit() {
-            return false;
-        }
-    }
-    true
-}