Give photo its command line back, and cover the parser that lost it

photo/cli.py

@@ -224,8 +224,6 @@ def main(argv: list[str] | None = None) -> int:
     c.add_argument("--ext-coeff", type=float, default=None, help="override epsilon, M^-1 cm^-1")
     c.add_argument("--quantum-yield", type=float, default=None)
     c.add_argument("--lifetime-ns", type=float, default=None)
-    c.add_argument(
-    )
     c.add_argument(
         "--excitation-coupling", type=float, default=None,
         help="transmission-weighted mean absorption over the delivered band "

tests/test_cli_parsers.py

@@ -0,0 +1,45 @@
+"""Every `*/cli.py` can build its parser and print `--help`.
+
+**Nothing in this suite did that, and a lens lost its command line for it.**
+`photo/cli.py` raised `TypeError: _ActionsContainer._get_positional_kwargs()
+missing 1 required positional argument: 'dest'` on *every* invocation,
+`--help` included, from the moment `--bleach-photons` was removed with G10: the
+deletion took the argument's body and left `c.add_argument(\n    )` behind.
+
+The 1210 tests around it all passed, because they call `evaluate()` and the
+check functions directly. An `argparse` mistake is not reachable that way --
+the parser is only built inside `main()`, and `main()` is only reached from a
+shell. So the defect was invisible to the suite and obvious to anyone who typed
+the command.
+
+`--help` is enough. `argparse` constructs every argument while building the
+parser, so a malformed one fails there rather than at parse time, and
+`SystemExit(0)` from `--help` is the cheapest proof that construction
+completed. This does not test what any CLI *does* -- the gate tests do that.
+"""
+
+import runpy
+import sys
+from pathlib import Path
+
+import pytest
+
+ROOT = Path(__file__).resolve().parent.parent
+
+#: Discovered rather than listed, so a new lens's CLI is covered the day it
+#: lands instead of the day someone remembers this file.
+CLIS = sorted(p.parent.name for p in ROOT.glob("*/cli.py"))
+
+
+def test_the_discovery_found_the_clis():
+    assert len(CLIS) >= 10, CLIS
+    assert "photo" in CLIS
+
+
+@pytest.mark.parametrize("package", CLIS)
+def test_cli_help_exits_cleanly(package, monkeypatch, capsys):
+    monkeypatch.setattr(sys, "argv", [package, "--help"])
+    with pytest.raises(SystemExit) as exc:
+        runpy.run_module(f"{package}.cli", run_name="__main__")
+    assert exc.value.code == 0, f"{package} --help exited {exc.value.code}"
+    assert "usage" in capsys.readouterr().out.lower()