JsonlBackwardIter: use memrchr3 for SIMD-accelerated scanning

Replaces byte-by-byte backward iteration with memrchr3('{', '}', '"')
which uses SIMD to jump between structurally significant bytes. Major
speedup on large transcripts (1.4GB+).

Also simplifies tail_messages to use a byte budget (200KB) instead
of token counting.

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

poc-memory/src/agents/defs.rs

@@ -456,7 +456,7 @@ fn resolve_conversation() -> String {
     let Some(path) = transcript else { return String::new() };
     let path_str = path.to_string_lossy();
 
-    let messages = crate::transcript::tail_messages(&path_str, 25_000);
+    let messages = crate::transcript::tail_messages(&path_str, 200_000);
     if messages.is_empty() { return String::new(); }
 
     let cfg = crate::config::get();

poc-memory/src/transcript.rs

@@ -4,6 +4,7 @@
 // and compaction detection. Used by memory-search (hook mode) and
 // parse-claude-conversation (debug tool).
 
+use memchr::memrchr3;
 use memmap2::Mmap;
 use serde_json::Value;
 use std::fs;
@@ -12,8 +13,10 @@ use std::path::Path;
 /// Scan backwards through mmap'd bytes, yielding byte slices of complete
 /// top-level JSON objects (outermost { to matching }).
 ///
-/// Tracks brace depth, skipping braces inside JSON strings. Returns
-/// objects in reverse order (newest first).
+/// Uses memrchr3 (SIMD) to jump between structurally significant bytes
+/// ({, }, ") instead of scanning byte-by-byte. Tracks brace depth,
+/// skipping braces inside JSON strings. Returns objects in reverse order
+/// (newest first).
 pub struct JsonlBackwardIter<'a> {
     data: &'a [u8],
     pos: usize,
@@ -29,17 +32,14 @@ impl<'a> Iterator for JsonlBackwardIter<'a> {
     type Item = &'a [u8];
 
     fn next(&mut self) -> Option<Self::Item> {
-        if self.pos == 0 {
-            return None;
-        }
-
-        // Find the closing } of the next object (scanning backward)
+        // Find the closing } of the next object
         let close = loop {
-            if self.pos == 0 { return None; }
-            self.pos -= 1;
-            if self.data[self.pos] == b'}' {
-                break self.pos;
+            let p = memrchr3(b'{', b'}', b'"', &self.data[..self.pos])?;
+            self.pos = p;
+            if self.data[p] == b'}' {
+                break p;
             }
+            // Skip past any { or " that aren't our closing brace
         };
 
         // Track brace depth to find matching {
@@ -47,22 +47,22 @@ impl<'a> Iterator for JsonlBackwardIter<'a> {
         let mut in_string = false;
 
         loop {
-            if self.pos == 0 {
-                return None;
-            }
-            self.pos -= 1;
-            let ch = self.data[self.pos];
+            let p = memrchr3(b'{', b'}', b'"', &self.data[..self.pos])?;
+            self.pos = p;
+            let ch = self.data[p];
 
             if in_string {
                 if ch == b'"' {
+                    // Check for escaped quote (count preceding backslashes)
                     let mut bs = 0;
-                    while self.pos > bs && self.data[self.pos - 1 - bs] == b'\\' {
+                    while p > bs + 1 && self.data[p - 1 - bs] == b'\\' {
                         bs += 1;
                     }
                     if bs % 2 == 0 {
                         in_string = false;
                     }
                 }
+                // { and } inside strings don't affect depth
                 continue;
             }
 
@@ -145,18 +145,17 @@ fn contains_bytes(haystack: &[u8], needle: &[u8]) -> bool {
 /// Reverse-scan a transcript file, collecting user/assistant messages
 /// until `max_tokens` tokens (~4 chars each) are accumulated. Stops at
 /// the last compaction boundary. Returns messages in chronological order.
-pub fn tail_messages(path: &str, max_tokens: usize) -> Vec<(String, String, String)> {
+pub fn tail_messages(path: &str, max_bytes: usize) -> Vec<(String, String, String)> {
     let (mmap, _file) = match mmap_transcript(path) {
         Some(v) => v,
         None => return Vec::new(),
     };
 
-    let compaction_marker = b"This session is being continued";
     let mut messages: Vec<(String, String, String)> = Vec::new();
-    let mut token_count = 0;
+    let mut total_bytes = 0;
 
     for obj_bytes in JsonlBackwardIter::new(&mmap) {
-        if token_count >= max_tokens { break; }
+        if total_bytes >= max_bytes { break; }
 
         // Quick byte check: skip objects that aren't user/assistant
         // (avoids parsing large tool_result / system objects)
@@ -197,7 +196,7 @@ pub fn tail_messages(path: &str, max_tokens: usize) -> Vec<(String, String, Stri
             .unwrap_or("")
             .to_string();
 
-        token_count += text.len() / 4;
+        total_bytes += text.len();
         messages.push((msg_type.to_string(), text, timestamp));
     }