fix: reject line breaks in frontmatter header values; note reference-impl limits

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

smolmaild.py

@@ -411,7 +411,13 @@ class Store:
 
 
 class RateLimiter:
-    """Fixed-window counter, keyed by IP address or by accept token (§10)."""
+    """Fixed-window counter, keyed by IP address or by accept token (§10).
+
+    A fixed window admits up to 2x the limit across a window boundary (a burst
+    at the end of one window plus a burst at the start of the next). Acceptable
+    for a reference server; a production limiter should use a sliding window or
+    token bucket.
+    """
 
     def __init__(self, limit: int, window: float = 60.0) -> None:
         self.limit = limit
@@ -776,6 +782,10 @@ def static_public(static_key: bytes) -> bytes:
 
 
 class MailServer(socketserver.ThreadingTCPServer):
+    # One thread per connection with no concurrency cap: only conn_limiter (per
+    # IP) bounds it, so a distributed connection flood can exhaust threads. The
+    # Reticulum server caps links (§13.8); a production TCP server should bound
+    # workers likewise, e.g. a thread pool or a max-connections gate.
     allow_reuse_address = True
     daemon_threads = True