COD-144 flush queued SSE output on empty buffer-load so new shells paint immediately

A freshly created shell session rendered blank until a tab-switch. selectSession()
fetches the terminal buffer, but for a just-started shell that fetch resolves before
the PTY emits its prompt, so the buffer is empty; the prompt then arrives as a live
SSE event queued during the load and _finishBufferLoad() discarded it. The discard is
correct for an established session (its fetched buffer already contains that output),
but harmful when the load painted nothing.

_finishBufferLoad(owner, { flushQueued }) now REPLAYS the queued events through
batchTerminalWrite (after _isLoadingBuffer is cleared, so they write through, not
re-queue) instead of discarding. selectSession passes flushQueued only in the empty
branch (no fresh buffer + no cache), so the established-session de-dup path is
unchanged. TDD: test/terminal-buffer-flush.test.ts exercises the real begin/finish
mixin (vm-harness, no jsdom).

src/web/public/app.js

@@ -3733,6 +3733,9 @@ class CodemanApp {
     // the buffer write, causing 70KB+ single-frame flushes that stall WebGL.
     // chunkedTerminalWrite also sets this, but we need it before the fetch too.
     const bufferLoadOwner = this._beginBufferLoad(selectGen);
+    // COD-144: track whether the load painted nothing (empty fetch + no cache).
+    // For that just-created-session case we flush (not discard) queued SSE events.
+    let bufferWasEmpty = false;
     try {
       // Fit terminal to container BEFORE writing any buffer data.
       // If the browser was resized while viewing another session, the terminal
@@ -3889,13 +3892,16 @@ class CodemanApp {
       } else if (!cachedBuffer) {
         // No fresh buffer and no cache — clear any stale content
         this._resetTerminalForReplay();
+        bufferWasEmpty = true;
       }
 
-      // Buffer load complete — unblock live SSE writes (queued events are discarded
-      // to prevent duplicate content). chunkedTerminalWrite calls _finishBufferLoad
-      // internally, but if we skipped the write (cache hit or empty), call it here.
+      // Buffer load complete — unblock live SSE writes. chunkedTerminalWrite calls
+      // _finishBufferLoad internally (discarding queued events to prevent duplicate
+      // content); if we skipped the write (cache hit or empty), call it here.
+      // COD-144: when the load painted nothing, FLUSH the queued events instead of
+      // discarding — a new session's prompt arrives only as a queued SSE event.
       if (this._isLoadingBuffer) {
-        this._finishBufferLoad(bufferLoadOwner);
+        this._finishBufferLoad(bufferLoadOwner, { flushQueued: bufferWasEmpty });
       }
       // Drop the guard so user input clears state normally
       this._restoringFlushedState = false;

src/web/public/terminal-ui.js

@@ -1925,11 +1925,25 @@ Object.assign(CodemanApp.prototype, {
    * Complete a buffer load: unblock live SSE writes.
    * Called when chunkedTerminalWrite finishes (or is skipped for empty buffers).
    *
-   * Queued SSE events are DISCARDED, not flushed. The loaded buffer from the API
-   * is the source of truth up to the response timestamp. SSE events queued during
-   * the fetch+write overlap with the buffer — flushing them writes duplicate data
-   * (especially Ink cursor-up redraws), corrupting the terminal display.
+   * By default queued SSE events are DISCARDED, not flushed. For an established
+   * session the loaded buffer from the API is the source of truth up to the
+   * response timestamp; SSE events queued during the fetch+write overlap already
+   * appear in that buffer, so flushing them writes duplicate data (especially Ink
+   * cursor-up redraws), corrupting the terminal display.
+   *
+   * COD-144: a brand-new session is the exception. Its terminal fetch can resolve
+   * BEFORE the PTY emits its first prompt, so the fetched buffer is empty and the
+   * prompt arrives only as a queued SSE event. Discarding it leaves the terminal
+   * blank until a tab-switch re-fetches a now-populated buffer. When the caller
+   * knows the load painted nothing (empty fetch + no cache), it passes
+   * `{ flushQueued: true }` so the queued events are REPLAYED through
+   * `batchTerminalWrite()` instead of dropped. Replay runs after `_isLoadingBuffer`
+   * is cleared, so the events write through normally and are not re-queued.
+   *
    * After unblocking, new SSE/WS events deliver subsequent output normally.
+   *
+   * @param {string} [owner] Load token from `_beginBufferLoad`; a stale owner is a no-op.
+   * @param {{ flushQueued?: boolean }} [opts] When `flushQueued` is true, replay any queued events.
    */
   _beginBufferLoad(owner) {
     if (this._bufferLoadSeq === undefined) this._bufferLoadSeq = 0;
@@ -1940,13 +1954,21 @@ Object.assign(CodemanApp.prototype, {
     return loadOwner;
   },
 
-  _finishBufferLoad(owner) {
+  _finishBufferLoad(owner, opts) {
     if (owner !== undefined && this._bufferLoadOwner !== owner) {
       return false;
     }
+    const queued = this._loadBufferQueue;
     this._isLoadingBuffer = false;
     this._loadBufferQueue = null;
     this._bufferLoadOwner = null;
+    // COD-144: replay (rather than discard) queued live events when the load
+    // painted nothing — the queued prompt is the only content a new session has.
+    if (opts?.flushQueued && queued && queued.length) {
+      for (const data of queued) {
+        this.batchTerminalWrite(data);
+      }
+    }
     return true;
   },