add position field to capnp schema

Position was only in the bincode cache (serde field) — it would
be lost on cache rebuild from capnp logs. Now persisted in the
append-only log via ContentNode.position @19.

Also fixes journal-tail sorting to extract dates from content
headers, falling back to key-embedded dates.

schema/memory.capnp

@@ -31,6 +31,9 @@ struct ContentNode {
   # Spaced repetition
   lastReplayed              @17 :Float64;   # unix epoch
   spacedRepetitionInterval  @18 :UInt32;    # days: 1, 3, 7, 14, 30
+
+  # Section ordering within a file
+  position  @19 :UInt32;    # 0 = file-level, 1+ = section index
 }
 
 enum NodeType {