Pin what actually keeps the probe config stable

`apply_arrival` carried a comment claiming `format-pyproject` expands inline
tables back into sections, and that writing sections avoided a ping-pong.
Running the formatter over a real `pyproject.toml` shows the reverse: it
collapses both shapes into dotted keys under `[tool.repomatic]`.

The rationale was wrong; the behaviour happens to be right. tomlrt cannot emit
a dotted key at all, since assigning one produces a quoted key holding dots,
so the formatter necessarily has the last word on layout. What keeps the two
from rewriting each other is that the dotted shape reads back as the same
nested value, so a second apply finds the label already declared and writes
nothing.

That is the invariant worth holding, and nothing held it. The new test applies
against the formatter's real output rather than against what the applier
writes, which is the version of this check that would have caught the mistaken
comment in the first place.

Author: kdeldycke

repomatic/runner_images.py

@@ -727,11 +727,17 @@ def apply_arrival(change: RunnerChange, pyproject_path: Path) -> bool:
     if not pyproject_path.is_file():
         return False
     doc = tomlrt.loads(pyproject_path.read_text(encoding="UTF-8"))
-    # Seeded through `Table` rather than a bare dict: `setdefault` with a dict
-    # writes an inline table, which `format-pyproject` then expands back into
-    # sections. The two would rewrite each other on every push, which is the
-    # generator/formatter ping-pong `claude.md` § Common maintenance pitfalls
-    # warns about.
+    # Seeded through `Table` rather than a bare dict, which `setdefault` would
+    # turn into an inline table.
+    #
+    # Either way `format-pyproject` normalizes what lands here into dotted keys
+    # under `[tool.repomatic]` (`test-matrix.variations.os = [ … ]`), and tomlrt
+    # cannot emit that form: assigning a dotted string produces a *quoted* key
+    # holding dots, which is a different key. So the formatter gets the last
+    # word on layout, and the invariant that matters is not the shape written
+    # but that re-reading the formatter's shape finds the label already there.
+    # It does, so the two converge instead of ping-ponging, per `claude.md`
+    # § Common maintenance pitfalls. `tests/test_runner_sync.py` pins that.
     node: Any = doc
     for key in ("tool", "repomatic", "test-matrix"):
         if key not in node:

tests/test_runner_sync.py

@@ -183,6 +183,36 @@ def test_apply_arrival_writes_sections_not_inline_tables(tmp_path) -> None:
     assert not apply_arrival(change, pyproject), "second run should be a no-op"
 
 
+def test_apply_arrival_is_idempotent_against_the_formatter_shape(tmp_path) -> None:
+    """Re-reading what `format-pyproject` leaves behind finds the probe present.
+
+    The applier writes sections; the formatter normalizes them into dotted keys
+    under `[tool.repomatic]`, and tomlrt cannot emit that form itself. So the
+    formatter gets the last word on layout, and what stops the two rewriting
+    each other on every push is this: the dotted shape reads back as the same
+    nested value, so a second apply finds the label already there and writes
+    nothing.
+
+    The fixture is the formatter's real output, captured from running
+    `pyproject-fmt` over a freshly written probe.
+    """
+    pyproject = tmp_path / "pyproject.toml"
+    pyproject.write_text(
+        '[project]\nname = "demo"\n\n'
+        "[tool.repomatic]\n"
+        'test-matrix.unstable = [ { os = "ubuntu-26.04" } ]\n'
+        'test-matrix.variations.os = [ "ubuntu-26.04" ]\n',
+        encoding="UTF-8",
+    )
+    (change,) = plan_runner_changes({}, {"ubuntu-24.04"}, [ARRIVAL], CATALOG)
+
+    assert not apply_arrival(change, pyproject), (
+        "the probe is already declared in the formatter's dotted-key shape, so "
+        "re-applying must write nothing: otherwise sync-runner-images and "
+        "format-pyproject each undo the other, forever"
+    )
+
+
 def test_apply_axes_retirement_moves_the_curated_tuple(tmp_path) -> None:
     """The axes move as text, keeping their comments and their ordering."""
     axes = tmp_path / "matrix_axes.py"