feat(IMP-08): U2 — aligner canonical sub-id + N-R5 decimal alias guard

align_sections_to_v4_granularity now emits canonical sub-section ids
of the form ${section_id}-sub-${ordinal} (e.g., "04-2-sub-1"), matching
the frontend drag/drop schema. Each drilled sub-section populates
heading_number (decimal "2.1" / integer "1" / None for undecorated)
and v4_alias_keys for legacy V4 keys.

N-R5 decimal-only alias guard : v4_alias_keys is populated only when
heading_number matches re.fullmatch(r"\d+\.\d+", ...). Integer-only
H3 headings (e.g., MDX 05's "### 1", "### 2") and bare H3 headings
produce no alias to avoid sibling-parent V4 collisions (RULE 0
generalization — applies to all 32-frame MDX, not MDX 05-specific).

The drill regex is broadened from r"^###\s+(\d+\.\d+)\s+..." to
r"^###\s+(?:(\d+(?:\.\d+)?)\s+)?(.+?)$" so integer-only and bare H3
headings are now recognised as sub-sections; they previously failed
the regex and were silently kept under the parent section.

Tests : 7 new cases (MdxSection default 4-positional callers, V4 exact
passthrough, decimal drill with alias, integer-only no-alias guard,
bare H3 no-alias, no-H3 passthrough, end-to-end aligner -> resolver
round-trip with legacy V4 alias). 15/15 in test_phase_z2_subsection_schema
+ 14 override + 8 fallback baseline = 37/37 PASS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/phase_z2_pipeline.py

@@ -375,6 +375,16 @@ def load_v4_result() -> dict:
 def align_sections_to_v4_granularity(sections: list[MdxSection], v4: dict) -> list[MdxSection]:
     """V4 section granularity 에 맞춰 sections 조정.
 
+    IMP-08 B-3 : canonical sub-section id ``${section_id}-sub-${ordinal}``
+    (예 : ``04-2-sub-1``) 를 emit 하고, legacy V4 키 (``04-2.1``) 는
+    ``v4_alias_keys`` 로 보존하여 ``_resolve_v4_section_key`` 가 alias 경로로
+    매칭한다. canonical ordinal id 는 frontend drag/drop override 와 동일
+    schema (`section_id-sub-N`).
+
+    N-R5 alias guard : heading_number 가 decimal (``2.1``) 일 때만 alias
+    emit. integer-only (``1``) / non-numeric heading 은 alias 0 — sibling
+    parent V4 evidence 로 잘못 promote 되는 collision 방지 (RULE 0).
+
     각 section 에 대해 :
       - V4 에 section.section_id 키 있음 → 그대로 유지 (## level 매칭)
       - V4 에 키 없고 raw_content 에 ### sub-section 존재 → ### 로 drill
@@ -388,31 +398,52 @@ def align_sections_to_v4_granularity(sections: list[MdxSection], v4: dict) -> li
     v4_keys = set(v4.get("mdx_sections", {}).keys())
     aligned: list[MdxSection] = []
 
+    # IMP-08 B-3 : capture optional heading-number prefix (decimal "2.1" or
+    # integer "1") + heading title. None group = bare "### Title".
+    sub_pattern = re.compile(
+        r"^###\s+(?:(\d+(?:\.\d+)?)\s+)?(.+?)$", re.MULTILINE
+    )
+    decimal_re = re.compile(r"\d+\.\d+")
+
     for section in sections:
         if section.section_id in v4_keys:
             aligned.append(section)
             continue
 
-        # ### drill 시도
-        sub_pattern = re.compile(r"^###\s+(\d+\.\d+)\s+(.+?)$", re.MULTILINE)
         sub_matches = list(sub_pattern.finditer(section.raw_content))
         if not sub_matches:
             aligned.append(section)  # drill 불가, V4 lookup 에서 abort 됨
             continue
 
-        # ### sub-section 추출
         mdx_id = section.section_id.split("-")[0]   # e.g., "04"
-        for i, m in enumerate(sub_matches):
-            subnum = m.group(1)                       # e.g., "2.1"
+        for ordinal, m in enumerate(sub_matches, start=1):
+            heading_number = m.group(1)  # decimal "2.1" / integer "1" / None
             sub_title = m.group(2).strip()
             start = m.end()
-            end = sub_matches[i + 1].start() if i + 1 < len(sub_matches) else len(section.raw_content)
+            end = (
+                sub_matches[ordinal].start()
+                if ordinal < len(sub_matches)
+                else len(section.raw_content)
+            )
             raw = section.raw_content[start:end].strip()
+
+            # N-R5 : alias only for decimal heading numbers. integer-only
+            # H3 (`### 1`) or undecorated H3 produce no alias to avoid
+            # sibling-parent V4 collisions (e.g., 05.mdx integer H3s).
+            alias_keys: list[str] = []
+            if heading_number and decimal_re.fullmatch(heading_number):
+                alias_keys.append(f"{mdx_id}-{heading_number}")
+
+            title = (
+                f"{heading_number} {sub_title}" if heading_number else sub_title
+            )
             aligned.append(MdxSection(
-                section_id=f"{mdx_id}-{subnum}",      # e.g., "04-2.1"
+                section_id=f"{section.section_id}-sub-{ordinal}",
                 section_num=section.section_num,
-                title=f"{subnum} {sub_title}",
+                title=title,
                 raw_content=raw,
+                heading_number=heading_number,
+                v4_alias_keys=alias_keys,
             ))
 
     return aligned