fix(memory): prevent infinite loop in chunk_text when overlap rewinds start (#3027)

When rfind("\n\n") found a match near the beginning of the sliding window,
`end` advanced by fewer bytes than CHUNK_OVERLAP_CHARS. The subsequent
`end.saturating_sub(CHUNK_OVERLAP_CHARS)` produced a value less than the
current `start`, and `ceil_char_boundary` returned a position behind it.
The old safeguard (`if start >= end`) did not fire because new_start <
start < end, so `start` regressed and the loop ran forever.

Fix: guarantee forward progress by taking `end` when `new_start <= start`.

This was the root cause of 100% CPU / GB RAM consumption at startup: the
embed backfill spawned background tasks for unembedded messages (including
an 85 KB tool_result), all of which entered the infinite loop simultaneously,
saturating all tokio worker threads and blocking log flushing.

Author: bug-ops

crates/zeph-memory/src/semantic/recall.rs

@@ -49,12 +49,12 @@ fn chunk_text(text: &str) -> Vec<&str> {
         if end >= text.len() {
             break;
         }
-        // Next chunk starts with overlap.
+        // Next chunk starts with overlap, but must always advance past the
+        // current position to prevent infinite loops when rfind finds a match
+        // very early in the slice (end barely advances, overlap rewinds start).
         let next = end.saturating_sub(CHUNK_OVERLAP_CHARS);
-        start = text.ceil_char_boundary(next);
-        if start >= end {
-            start = end; // safeguard against infinite loop
-        }
+        let new_start = text.ceil_char_boundary(next);
+        start = if new_start > start { new_start } else { end };
     }
 
     chunks