refactor: keep root install as the sole installer source

test/onboarding/install_support.py

@@ -0,0 +1,32 @@
+"""Read the real installer for focused tests; no installation logic is copied."""
+
+from pathlib import Path
+from types import ModuleType
+
+
+ROOT = Path(__file__).resolve().parents[2]
+ENTRY = ROOT / "install"
+
+
+def entry_parts():
+    entry = ENTRY.read_text()
+    opening = "<<'DEVER_INSTALL_PYTHON'\n"
+    closing = "\nDEVER_INSTALL_PYTHON\n"
+    if entry.count(opening) != 1 or entry.count(closing) != 1:
+        raise ValueError("root install must contain exactly one Python heredoc")
+    before, remainder = entry.split(opening)
+    source, after = remainder.split(closing)
+    return before + opening, source, closing + after
+
+
+def load_installer():
+    _, source, _ = entry_parts()
+    module = ModuleType("dever_installer")
+    exec(compile(source, str(ENTRY) + "::python", "exec"), module.__dict__)
+    return module
+
+
+def entry_with_python(probe):
+    """Keep the actual shell wrapper while a harmless probe observes its contract."""
+    before, _, after = entry_parts()
+    return before + probe.rstrip("\n") + after