feat: stdio server spawn-test (issue #2)

Add spawn_test() to bcc_core — spawns a stdio server for up to 3 s,
captures stderr, and reports ok/exited/crashed/not_found. Key design
decisions driven by real MCP server behaviour:

- stdin=PIPE (never written): servers block on JSON-RPC input and stay
  alive, so "still running after timeout" reliably signals a healthy
  start. stdin=DEVNULL would send EOF, causing well-behaved servers to
  exit 0 and be misclassified as "exited".
- Command resolved via shutil.which(augmented_path()) before Popen so
  subprocess PATH resolution is unambiguous across platforms.
- start_new_session=True on POSIX + os.killpg on timeout: kills the
  whole process group, not just the launcher (npx, uvx), which would
  otherwise orphan the actual node/python grandchild process.
- stdout=DEVNULL: draining a PIPE we don't read would deadlock at ~64 KB.
- stderr drained in a daemon thread, capped at 4 KB.

GUI: SpawnTester(QThread) wraps spawn_test; "Test launch" button in
ServerEditor dep row (stdio only, visible when command resolves ok/warn).
Result colours match the existing dep-status palette (green/amber/red).
Stderr appended to the diagnostics panel if it is open.

7 new unit tests cover all outcomes and the stdin-open regression guard.
Ran python bcc.py locally: button appears for stdio servers whose command
resolves, is hidden for remote servers and missing-command servers.

Closes #2

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

bcc.py

@@ -132,6 +132,19 @@ class ConnTester(QThread):
         self.done.emit(ok, detail)
 
 
+class SpawnTester(QThread):
+    done = Signal(dict)
+
+    def __init__(self, data, timeout=3.0):
+        super().__init__()
+        self._data = data
+        self._timeout = timeout
+
+    def run(self):
+        result = core.spawn_test(self._data, self._timeout)
+        self.done.emit(result)
+
+
 # --------------------------------------------------------------------------- #
 # Small reusable: key/value editor (for env and headers)
 # --------------------------------------------------------------------------- #
@@ -354,6 +367,10 @@ class ServerEditor(QFrame):
         self.test_btn = QPushButton("Test connection")
         self.test_btn.clicked.connect(self._test_remote)
         self.test_btn.setVisible(False)
+        self.spawn_btn = QPushButton("Test launch")
+        self.spawn_btn.setToolTip("Spawn the server for 3 s and report whether it starts cleanly")
+        self.spawn_btn.clicked.connect(self._test_spawn)
+        self.spawn_btn.setVisible(False)
         self.details_btn = QPushButton("Details ▸")
         self.details_btn.setCheckable(True)
         self.details_btn.toggled.connect(self._toggle_diag)
@@ -363,6 +380,7 @@ class ServerEditor(QFrame):
         dep.addWidget(self.dep_label, 1)
         dep.addWidget(self.fix_btn)
         dep.addWidget(self.test_btn)
+        dep.addWidget(self.spawn_btn)
         dep.addWidget(self.details_btn)
         dep.addWidget(recheck)
         outer.addLayout(dep)
@@ -585,7 +603,9 @@ class ServerEditor(QFrame):
         res = core.check_dependency(data)
         self._set_dep(res)
         self.fix_btn.setVisible(res["status"] == "warn")
-        self.test_btn.setVisible(self.type.currentIndex() == 1)
+        is_remote = self.type.currentIndex() == 1
+        self.test_btn.setVisible(is_remote)
+        self.spawn_btn.setVisible(not is_remote and res["status"] in ("ok", "warn"))
         problem = res["status"] in ("missing", "warn", "unknown")
         if auto_open and problem and not self.details_btn.isChecked():
             self.details_btn.setChecked(True)  # opens panel (fills text via _toggle_diag)
@@ -650,6 +670,40 @@ class ServerEditor(QFrame):
         self.dep_label.setText(f"reachable · {detail}" if ok else f"unreachable · {detail}")
         self.dep_label.setStyleSheet(f"color: {color};")
 
+    def _test_spawn(self):
+        self.spawn_btn.setEnabled(False)
+        self.spawn_btn.setText("Launching…")
+        self.dep_dot.setText("◆")
+        self.dep_dot.setStyleSheet(f"color: {MUTED}; font-size: 14px;")
+        self.dep_label.setText("spawning server…")
+        self.dep_label.setStyleSheet(f"color: {MUTED};")
+        self._spawner = SpawnTester(self.dump_data(), timeout=3.0)
+        self._spawner.done.connect(self._on_spawn_done)
+        self._spawner.start()
+
+    def _on_spawn_done(self, result: dict):
+        self.spawn_btn.setEnabled(True)
+        self.spawn_btn.setText("Test launch")
+        outcome = result.get("outcome", "")
+        detail = result.get("detail", "")
+        stderr = result.get("stderr", "").strip()
+        if outcome == "ok":
+            color = GOOD
+            label = f"started · {detail}"
+        elif outcome == "exited":
+            color = WARN
+            label = f"exited cleanly · {detail}"
+        else:
+            color = BAD
+            label = f"{outcome} · {detail}"
+        self.dep_dot.setText("●")
+        self.dep_dot.setStyleSheet(f"color: {color}; font-size: 14px;")
+        self.dep_label.setText(label)
+        self.dep_label.setStyleSheet(f"color: {color};")
+        if stderr and self.diag_card.isVisible():
+            current = self.diag_text.toPlainText()
+            self.diag_text.setPlainText(current + f"\n\n── stderr from Test launch ──\n{stderr}")
+
 
 # --------------------------------------------------------------------------- #
 # Arguments editor: one line = one argument, with a numbered gutter so that