test(kernsheet): give _FakeKern a spine_count for the new review

The spine_count review (481e114) reads kern.spine_count, but the editor
tests' _FakeKern stub lacked it, so every editor test exercising reviews
hit AttributeError. Add spine_count (default 2 = a normal 2-staff scan,
which doesn't trip the review). Test-only; production KernReader already
exposes spine_count.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A3yZYjbFTdhgQNyTXTtM1F

Author: anselm67

tests/kernsheet/test_editor.py

@@ -57,11 +57,13 @@ def __init__(
         first_bar: int = 1,
         bar_zero: bool = False,
         text: list[str] | None = None,
+        spine_count: int = 2,
     ) -> None:
         self.bar_count = bar_count
         self.first_bar = first_bar
         self._bar_zero = bar_zero
         self._text = text
+        self.spine_count = spine_count
 
     def has_bar_zero(self) -> bool:
         return self._bar_zero