Mark roadmap A3 (error surfacing) done

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ROADMAP.md

@@ -21,10 +21,10 @@ it exists. Checkboxes track status.
       the retained log is bounded, and `wait_end` runs once. Verified a 2MB-
       output child completes with the main loop never running.
 
-- [ ] **A3. Surface real errors to the user.** Failures show a generic error
-      window; the actual failing command + stderr is buried in the log. On
-      failure, show the command and the last N stderr lines. (Diagnosing the
-      deadlock this session required strace/`/proc` spelunking.)
+- [x] **A3. Surface real errors to the user.** DONE — the error dialog now
+      leads with a focused summary (failed step, its command, last stderr
+      lines) built from the executor's bounded `stderr_data`, with the full log
+      still in the expander. Markup-escaped + unit-tested.
 
 - [ ] **A4. Narrow bare `except:` blocks.** Config loading, `pre_function`/
       `post_function`, ffprobe JSON parsing, etc. use `except: pass`, silently