transcript: fix close-brace finder to track string boundaries

The backward JSON scanner (JsonlBackwardIter and TailMessages) was
matching } characters inside JSON strings — code blocks full of Rust
braces being the primary offender. This caused:

- Quadratic retry behavior on code-heavy transcripts (wrong object
  boundaries → serde parse failure → retry from different position)
- Inconsistent find_last_compaction_in_file offsets across calls,
  making detect_new_compaction fire repeatedly → context reload on
  every hook call → seen set growing without bound

Fix: add string-boundary tracking with escaped-quote handling to
the close-brace finder loop, matching the existing logic in the
depth-tracking loop.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-memory/src/transcript.rs

@@ -32,14 +32,31 @@ impl<'a> Iterator for JsonlBackwardIter<'a> {
     type Item = &'a [u8];
 
     fn next(&mut self) -> Option<Self::Item> {
-        // Find the closing } of the next object
+        // Find the closing } of the next object, skipping } inside strings
-        let close = loop {
+        let close = {
+            let mut in_string = false;
+            loop {
                 let p = memrchr3(b'{', b'}', b'"', &self.data[..self.pos])?;
                 self.pos = p;
-            if self.data[p] == b'}' {
+                let ch = self.data[p];
-                break p;
+
+                if in_string {
+                    if ch == b'"' {
+                        let mut bs = 0;
+                        while p > bs + 1 && self.data[p - 1 - bs] == b'\\' {
+                            bs += 1;
+                        }
+                        if bs % 2 == 0 { in_string = false; }
+                    }
+                    continue;
+                }
+
+                match ch {
+                    b'}' => break p,
+                    b'"' => in_string = true,
+                    _ => {}
+                }
             }
-            // Skip past any { or " that aren't our closing brace
         };
 
         // Track brace depth to find matching {
@@ -164,11 +181,31 @@ impl Iterator for TailMessages {
 
     fn next(&mut self) -> Option<Self::Item> {
         loop {
-            // Find closing }
+            // Find closing }, skipping } inside strings
-            let close = loop {
+            let close = {
+                let mut in_string = false;
+                loop {
                     let p = memrchr3(b'{', b'}', b'"', &self.mmap[..self.pos])?;
                     self.pos = p;
-                if self.mmap[p] == b'}' { break p; }
+                    let ch = self.mmap[p];
+
+                    if in_string {
+                        if ch == b'"' {
+                            let mut bs = 0;
+                            while p > bs + 1 && self.mmap[p - 1 - bs] == b'\\' {
+                                bs += 1;
+                            }
+                            if bs % 2 == 0 { in_string = false; }
+                        }
+                        continue;
+                    }
+
+                    match ch {
+                        b'}' => break p,
+                        b'"' => in_string = true,
+                        _ => {}
+                    }
+                }
             };
 
             // Track brace depth to find matching {