context: tighten timestamp schema; every AstNode has one

Previously NodeLeaf.timestamp and AstNode::Branch.timestamp accepted
null or missing via a deserialize_timestamp_or_epoch fallback — legacy
entries in conversation.jsonl from before Branch timestamps existed
(and from before chrono serialization was wired up) would load with
UNIX_EPOCH as a sentinel. Downstream, node_timestamp_ns() returned
Option<i64> and callers had to handle None as "old entry, skip."

That second filter was silently dropping every candidate in
score_finetune_candidates when scoring an older session — the F6
screen showed "0 above threshold" even when max_divergence was
orders of magnitude above the threshold, because every entry was
failing the None check, not the divergence check.

The fix, in three parts:

1. src/bin/fix-timestamps.rs — one-off migration tool that walks a
   conversation.jsonl, linearly interpolates timestamps for entries
   stuck at UNIX_EPOCH (using surrounding real timestamps as anchors),
   propagates to child leaves with per-sibling ns offsets, and bumps
   any collisions by 1 ns for uniqueness. Ran against the current
   session's log: 11887 entries, 72289 ns bumps, all unique.

2. context.rs — drop default_timestamp and
   deserialize_timestamp_or_epoch. NodeLeaf and Branch now require a
   present non-null timestamp on deserialize. Tests flip from
   "missing/null → UNIX_EPOCH" to "missing/null → Err."

3. subconscious/learn.rs — node_timestamp_ns now returns i64, not
   Option<i64>. The matching caller in score_finetune_candidates
   collapses from a Some/None match to a single trained-set check.
   mind/log.rs's oldest_timestamp no longer filters UNIX_EPOCH.

Every line currently on disk has already been migrated. Going
forward, new AstNodes always carry real timestamps (Utc::now() at
construction time), so the strict schema is the invariant, not an
aspiration.

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

src/mind/log.rs

@@ -55,17 +55,13 @@ impl ConversationLog {
     }
 
     pub fn oldest_timestamp(&self) -> Option<chrono::DateTime<chrono::Utc>> {
-        // 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()? };
         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() {
-                    let ts = leaf.timestamp();
-                    if ts != chrono::DateTime::UNIX_EPOCH {
-                        return Some(ts);
-                    }
+                    return Some(leaf.timestamp());
                 }
             }
         }