learn: nanosecond timestamps, token ranges for /score

Two related changes to the learn subsystem:

1. AST node timestamps are now non-optional — both Leaf and Branch
   variants carry a DateTime<Utc>. UNIX_EPOCH means "unset" (old entries
   deserialized from on-disk conversation logs).

   Training uses timestamps as unique keys for dedup, so we promote to
   nanosecond precision: node_timestamp_ns(), TrainData.timestamp_ns,
   FinetuneCandidate.timestamp_ns, mark_trained(ns).

2. build_token_ids() now also returns token-position ranges of assistant
   messages. These are passed to vLLM's /score endpoint via the new
   score_ranges field so only scored-position logprobs are returned —
   cuts bandwidth/compute when scoring small windows.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/mind/log.rs

@@ -55,15 +55,15 @@ impl ConversationLog {
     }
 
     pub fn oldest_timestamp(&self) -> Option<chrono::DateTime<chrono::Utc>> {
-        // Read forward from the start to find first timestamp
+        // Read forward from the start to find first non-epoch timestamp
         let file = File::open(&self.path).ok()?;
         let mmap = unsafe { Mmap::map(&file).ok()? };
-        // Find first { ... } and parse
         for line in mmap.split(|&b| b == b'\n') {
             if line.is_empty() { continue; }
             if let Ok(node) = serde_json::from_slice::<AstNode>(line) {
                 if let Some(leaf) = node.leaf() {
-                    if let Some(ts) = leaf.timestamp() {
+                    let ts = leaf.timestamp();
+                    if ts != chrono::DateTime::UNIX_EPOCH {
                         return Some(ts);
                     }
                 }