fix(changelog): make independent prepends union-mergeable

.gitattributes

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+CHANGELOG.md merge=union

CHANGELOG.d/20260911-changelog-union-contract.md

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+## Changed
+
+- `CHANGELOG.md`에 `merge=union` 속성을 추가하고, union 병합이 제목을 코드
+  펜스 안으로 삼키거나 빈 섹션을 만드는 경우를 감지하는 구조 계약을
+  추가했습니다. 현재 changelog의 무펜스 구조를 유지하면서 독립 PR의
+  prepend 충돌을 줄입니다.

tests/test_changelog_structure_contract.py

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+"""Structural guardrails for the union-merged changelog."""
+
+from pathlib import Path
+import re
+
+
+ROOT = Path(__file__).resolve().parents[1]
+CHANGELOG = ROOT / "CHANGELOG.md"
+ATTRIBUTES = ROOT / ".gitattributes"
+
+
+def test_changelog_uses_union_merge_driver() -> None:
+    """Prepending independent release notes must not create pairwise conflicts."""
+    assert "CHANGELOG.md merge=union" in ATTRIBUTES.read_text(encoding="utf-8")
+
+
+def test_changelog_sections_are_nonempty_and_fences_are_balanced() -> None:
+    """Union merging must not silently swallow a heading inside a code fence."""
+    lines = CHANGELOG.read_text(encoding="utf-8").splitlines()
+    headings = [index for index, line in enumerate(lines) if line.startswith("### ")]
+    assert headings
+    assert headings[0] == 0
+
+    in_fence = False
+    fence_count = 0
+    for line in lines:
+        if line.startswith("```"):
+            in_fence = not in_fence
+            fence_count += 1
+        if line.startswith("### "):
+            assert not in_fence
+    assert fence_count % 2 == 0
+
+    for start, end in zip(headings, [*headings[1:], len(lines)]):
+        body = [line for line in lines[start + 1 : end] if line.strip()]
+        assert body, f"empty changelog section at line {start + 1}"
+        assert re.match(r"^### .+", lines[start])