fix(variation_checker): measure longest run for consecutive same-size shots

Check 2 flagged 'N consecutive same-size shots' from a count of every equal
adjacent pair across the whole plan, not the length of any real run. So three
separate 2-shot groups (wide,wide,cu,cu,med,med) tripped a false '3
consecutive' violation, while a genuine run of 3 (only 2 pairs) was never
flagged. Track the current run length, reset on change, and compare the longest
run >= 3.

Adds regression tests: non-consecutive pairs pass, a true run of 3 is flagged,
unspecified shots don't form a run.

Closes #268

tests/lib/test_variation_checker_runs.py

@@ -0,0 +1,37 @@
+"""Regression test for check_scene_variation consecutive-run counting.
+
+The "consecutive same-size shots" check summed every equal adjacent pair across
+the whole plan instead of measuring the longest actual run, so an editorially
+varied plan of separate 2-shot groups (wide,wide,cu,cu,med,med) falsely tripped
+a "3 consecutive same-size shots" violation.
+"""
+
+import sys
+from pathlib import Path
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent.parent
+sys.path.insert(0, str(PROJECT_ROOT))
+
+from lib.variation_checker import check_scene_variation  # noqa: E402
+
+
+def _scenes(sizes):
+    return [{"shot_language": {"shot_size": s}} for s in sizes]
+
+
+def test_non_consecutive_same_size_pairs_do_not_trip_run_check():
+    # Three separate 2-shot groups — longest run is 2, not 3.
+    res = check_scene_variation(_scenes(["wide", "wide", "cu", "cu", "medium", "medium"]))
+    assert not any("consecutive same-size" in v for v in res["violations"])
+
+
+def test_true_run_of_three_is_flagged():
+    res = check_scene_variation(_scenes(["wide", "wide", "wide", "cu", "medium"]))
+    assert any("3 consecutive same-size" in v for v in res["violations"])
+
+
+def test_unspecified_shots_do_not_form_a_run():
+    res = check_scene_variation(
+        _scenes(["unspecified", "unspecified", "unspecified", "unspecified"])
+    )
+    assert not any("consecutive same-size" in v for v in res["violations"])