fix unwrap-on-partial_cmp, dedup helpers, O(1) relation dedup

Replace all partial_cmp().unwrap() with total_cmp() in spectral.rs
and knowledge.rs — eliminates potential panics on NaN without
changing behavior for normal floats.

Use existing weighted_distance() and eigenvalue_weights() helpers in
nearest_neighbors() and nearest_to_seeds() instead of inlining the
same distance computation.

Move parse_timestamp_to_epoch() from enrich.rs to util.rs — was
duplicated logic, now shared.

Replace O(n²) relation existence check in init_from_markdown() with
a HashSet of (source, target) UUID pairs. With 26K relations this
was scanning linearly for every link in every markdown unit.

poc-memory/src/util.rs

@@ -28,3 +28,18 @@ pub fn truncate(text: &str, max_len: usize, suffix: &str) -> String {
 pub fn first_n_chars(s: &str, n: usize) -> String {
     s.chars().take(n).collect()
 }
+
+/// Parse a timestamp string to unix epoch seconds.
+/// Handles: "2026-03-05T19:56:00", "2026-03-05T19:56", "2026-03-05 19:56:00", "2026-03-05 19:56"
+pub fn parse_timestamp_to_epoch(ts: &str) -> Option<i64> {
+    use chrono::{Local, NaiveDateTime, TimeZone};
+    let formats = ["%Y-%m-%dT%H:%M:%S", "%Y-%m-%dT%H:%M", "%Y-%m-%d %H:%M:%S", "%Y-%m-%d %H:%M"];
+    for fmt in &formats {
+        if let Ok(ndt) = NaiveDateTime::parse_from_str(ts, fmt) {
+            if let Some(dt) = Local.from_local_datetime(&ndt).earliest() {
+                return Some(dt.timestamp());
+            }
+        }
+    }
+    None
+}