feat(IMP-08): U1 — schema helper + V4 alias resolver (4 lookup sites)

Adds sub-section schema fields (heading_number / v4_alias_keys /
sub_sections) to MdxSection with defaults so existing 4-positional
constructions remain valid. Introduces _resolve_v4_section_key helper
that resolves a V4 mdx_sections key in exact > alias > None order with
no parent/sibling promotion (axis 7 hybrid lock).

Rewires four runtime V4 lookup sites (lookup_v4_match,
lookup_v4_match_with_fallback, lookup_v4_all_judgments,
lookup_v4_candidates) to accept an optional alias_keys kwarg and go
through the resolver. U1 callers pass empty alias lists so behaviour
is byte-identical to the previous exact-match path; U2 will populate
aliases from MDX heading_number metadata.

Closure callers in run_phase_z2 build section_alias_by_id from
MdxSection.v4_alias_keys and forward into lookup_fn /
candidates_lookup_fn / lookup_v4_all_judgments (Step 7-A trace) and
into _select_template_for_overrides single-section selector.

Step 9 candidate report (post-decision diagnostic) is marked with an
inline English exemption comment per N-R6 — runtime selection goes
through _resolve_v4_section_key, the report path stays a direct
dict-shape lookup to avoid debug_zones schema plumbing.

derive_parent_id now recognises canonical ordinal ids
("03-1-sub-2" -> "03-1") first and keeps the legacy decimal fallback
("04-2.1" -> "04-2") for V4 alias compatibility.

Tests : 8 synthetic cases in tests/test_phase_z2_subsection_schema.py
covering derive_parent_id ordinal/decimal/none and the resolver
exact/alias/no-promote/miss cases. 30/30 PASS combined with the 14
override + 8 fallback baseline.

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

src/phase_z2_composition.py

@@ -21,6 +21,7 @@ Pipeline 의 빠진 layer = MDX 덩어리들을 *최종 zone unit* 으로 묶는
 
 from __future__ import annotations
 
+import re
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Optional
@@ -371,13 +372,20 @@ class CompositionUnit:
 # ─── Heading Tree ──────────────────────────────────────────────
 
 def derive_parent_id(section_id: str) -> Optional[str]:
-    """section_id 에서 parent 도출 — V4 키 컨벤션 기반.
+    """Section id -> parent id derivation by V4 key convention.
 
-    예시 (코멘트, 룰 X) :
-      - "04-2.1" → "04-2"  (decimal suffix → strip)
-      - "04-1"   → None    (top-level, no parent)
-      - "04"     → None
+    IMP-08 B-3 : canonical ordinal `${parent}-sub-${n}` recognised first;
+    legacy decimal `04-2.1` kept as fallback alias path.
+
+    Examples (illustrative, not rules) :
+      - "03-1-sub-2" -> "03-1"  (canonical ordinal, IMP-08)
+      - "04-2.1"     -> "04-2"  (decimal suffix, legacy V4 key style)
+      - "04-1"       -> None    (top-level, no parent)
+      - "04"         -> None
     """
+    m = re.fullmatch(r"(.+?)-sub-(\d+)", section_id)
+    if m:
+        return m.group(1)
     parts = section_id.split("-", 1)
     if len(parts) != 2:
         return None