fix: spawn-test stderr persistence and drain-cap deadlock (PR #11 review)

Two bugs caught in supervisor review:

1. Stderr was silently dropped when the Details panel was closed during a
   crash. _on_spawn_done appended to diag_text only when the panel was
   already open, and refresh_dependency() clobbered that text on the next
   field change anyway.
   Fix: stash the result in self._last_spawn; _full_diag_text() appends
   the stderr section whenever diag text is generated; _on_spawn_done
   auto-opens the panel on non-ok outcomes (same pattern as the existing
   auto_open for missing commands).

2. _drain stopped reading once _STDERR_CAP (4 KB) was reached. A process
   that writes more than 4 KB then blocked on a full pipe buffer, never
   exited, and was misclassified as "ok" instead of "crashed".
   Fix: drain to EOF unconditionally; keep only the first _STDERR_CAP bytes.
   Regression test: 64 KB stderr + exit(3) → outcome "crashed".

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

bcc_core.py

@@ -1114,12 +1114,18 @@ def spawn_test(data: dict, timeout: float = 3.0) -> dict:
     stderr_chunks: list[bytes] = []
 
     def _drain(pipe) -> None:
+        # Read until EOF so the pipe buffer never fills and blocks the subprocess.
+        # Only keep the first _STDERR_CAP bytes; the rest is discarded.
+        captured = 0
         try:
-            while sum(len(c) for c in stderr_chunks) < _STDERR_CAP:
+            while True:
                 chunk = pipe.read(1024)
                 if not chunk:
                     break
-                stderr_chunks.append(chunk)
+                if captured < _STDERR_CAP:
+                    keep = min(len(chunk), _STDERR_CAP - captured)
+                    stderr_chunks.append(chunk[:keep])
+                    captured += keep
         except Exception:
             pass