fix: prevent stale terminal data on tab switch — add chunkedTerminalWrite cancellation

chunkedTerminalWrite used requestAnimationFrame to write buffer chunks across
frames but had no cancellation. When switching tabs, old session's remaining
chunks continued writing stale data into the new session's terminal, causing
visual artifacts and garbled content.

- Add _chunkedWriteGen generation counter to abort in-flight chunked writes
- Bump gen early in selectSession() and SSE reconnect to immediately cancel
- Guard finish() so aborted writes don't flush SSE queue for wrong session
- Add fitAddon.fit() before buffer writes to sync terminal dimensions
- Add fitAddon.fit() in sendResize() to ensure local/server dim parity

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/web/public/terminal-ui.js

@@ -219,6 +219,9 @@ Object.assign(CodemanApp.prototype, {
     // Welcome message
     this.showWelcome();
 
+    // Generation counter for chunkedTerminalWrite — aborts stale writes on tab switch
+    this._chunkedWriteGen = 0;
+
     // Handle resize with throttling for performance
     this._resizeTimeout = null;
     this._lastResizeDims = null;
@@ -1033,6 +1036,10 @@ Object.assign(CodemanApp.prototype, {
    * @returns {Promise<void>} - Resolves when all chunks written
    */
   chunkedTerminalWrite(buffer, chunkSize = TERMINAL_CHUNK_SIZE) {
+    // Generation counter: if a newer chunkedTerminalWrite starts (tab switch),
+    // older writes abort instead of continuing to push stale data into the terminal.
+    const writeGen = ++this._chunkedWriteGen;
+
     return new Promise((resolve) => {
       if (!buffer || buffer.length === 0) {
         this._finishBufferLoad();
@@ -1049,7 +1056,10 @@ Object.assign(CodemanApp.prototype, {
       const cleanBuffer = buffer.replace(DEC_SYNC_STRIP_RE, '');
 
       const finish = () => {
-        this._finishBufferLoad();
+        // Only finish if we're still the active write — a newer write owns buffer load state
+        if (this._chunkedWriteGen === writeGen) {
+          this._finishBufferLoad();
+        }
         resolve();
       };
 
@@ -1067,6 +1077,12 @@ Object.assign(CodemanApp.prototype, {
       const _chunkStart = performance.now();
       let _chunkCount = 0;
       const writeChunk = () => {
+        // Abort if a newer chunked write started (user switched tabs)
+        if (this._chunkedWriteGen !== writeGen) {
+          resolve();
+          return;
+        }
+
         if (offset >= cleanBuffer.length) {
           const _totalMs = performance.now() - _chunkStart;
           console.log(`[CRASH-DIAG] chunkedTerminalWrite complete: ${cleanBuffer.length} bytes in ${_chunkCount} chunks, ${_totalMs.toFixed(0)}ms total`);
@@ -1240,6 +1256,9 @@ Object.assign(CodemanApp.prototype, {
    * @returns {Promise<void>}
    */
   async sendResize(sessionId) {
+    // Fit terminal to container before reading dimensions — ensures local
+    // terminal size matches what we report to the server PTY.
+    if (this.fitAddon) this.fitAddon.fit();
     const dims = this.getTerminalDimensions();
     if (!dims) return;
     // Fast path: WebSocket resize