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>

tests/test_phase_z2_subsection_schema.py

@@ -7,11 +7,17 @@ NO MDX-specific section ids beyond canonical id format.
 Locked scope (Stage 3 R8) :
   A. ``derive_parent_id`` canonical ordinal recognition + legacy decimal fallback.
   B. ``_resolve_v4_section_key`` exact > alias > None (no parent/sibling promotion).
+  C. ``align_sections_to_v4_granularity`` canonical ordinal id emit + N-R5
+     decimal-only alias guard + MdxSection default-construction stability.
 """
 from __future__ import annotations
 
 from src.phase_z2_composition import derive_parent_id
-from src.phase_z2_pipeline import _resolve_v4_section_key
+from src.phase_z2_pipeline import (
+    MdxSection,
+    _resolve_v4_section_key,
+    align_sections_to_v4_granularity,
+)
 
 
 # ─── A. derive_parent_id ────────────────────────────────────────────────────
@@ -80,3 +86,95 @@ def test_alias_resolver_miss_returns_none():
         _resolve_v4_section_key(v4, "04-2-sub-1", alias_keys=["04-2.1"])
         is None
     )
+
+
+# ─── C. align_sections_to_v4_granularity ────────────────────────────────────
+
+
+def _section(section_id, num, title, raw_content):
+    """Build an MdxSection with default sub-section schema fields."""
+    return MdxSection(
+        section_id=section_id,
+        section_num=num,
+        title=title,
+        raw_content=raw_content,
+    )
+
+
+def test_mdx_section_default_construction_preserves_4_positional_callers():
+    # IMP-08 B-3 : MdxSection still accepts the legacy 4-positional shape
+    # (defaults for heading_number / v4_alias_keys / sub_sections).
+    s = MdxSection("04-1", 1, "1. Top", "body")
+    assert s.heading_number is None
+    assert s.v4_alias_keys == []
+    assert s.sub_sections == []
+
+
+def test_align_passthrough_when_v4_key_exact_match():
+    # Section already aligned to V4 key — aligner keeps it untouched.
+    sections = [_section("04-1", 1, "1. Top", "body")]
+    v4 = {"mdx_sections": {"04-1": {"judgments_full32": []}}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert len(out) == 1
+    assert out[0].section_id == "04-1"
+
+
+def test_align_drill_emits_canonical_ordinal_id_with_decimal_alias():
+    # Decimal H3 headings -> canonical ordinal id + decimal alias (legacy V4 key).
+    raw = "### 2.1 First\nbody1\n### 2.2 Second\nbody2\n"
+    sections = [_section("04-2", 2, "2. Parent", raw)]
+    v4 = {"mdx_sections": {}}  # forces drill (no exact key)
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert [s.section_id for s in out] == ["04-2-sub-1", "04-2-sub-2"]
+    assert [s.heading_number for s in out] == ["2.1", "2.2"]
+    # N-R5 : decimal headings -> alias emitted.
+    assert out[0].v4_alias_keys == ["04-2.1"]
+    assert out[1].v4_alias_keys == ["04-2.2"]
+
+
+def test_align_drill_integer_only_h3_emits_no_alias_n_r5_guard():
+    # N-R5 : integer-only H3 (e.g., "### 1 Title") must NOT generate an alias,
+    # otherwise it would collide with sibling parent V4 entries (`{mdx_id}-1`).
+    raw = "### 1 Alpha\nbody1\n### 2 Beta\nbody2\n"
+    sections = [_section("05-2", 2, "2. Parent", raw)]
+    v4 = {"mdx_sections": {}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert [s.section_id for s in out] == ["05-2-sub-1", "05-2-sub-2"]
+    assert [s.heading_number for s in out] == ["1", "2"]
+    assert out[0].v4_alias_keys == []
+    assert out[1].v4_alias_keys == []
+
+
+def test_align_drill_undecorated_h3_emits_no_alias():
+    # Plain `### Title` without numeric prefix -> heading_number=None, no alias.
+    raw = "### Alpha\nbody1\n### Beta\nbody2\n"
+    sections = [_section("03-3", 3, "3. Parent", raw)]
+    v4 = {"mdx_sections": {}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert [s.section_id for s in out] == ["03-3-sub-1", "03-3-sub-2"]
+    assert [s.heading_number for s in out] == [None, None]
+    assert all(s.v4_alias_keys == [] for s in out)
+
+
+def test_align_no_h3_passes_section_through_unchanged():
+    # No H3 sub-headings in raw_content -> aligner keeps the section.
+    sections = [_section("04-1", 1, "1. Top", "no subheadings here\njust prose")]
+    v4 = {"mdx_sections": {}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert len(out) == 1
+    assert out[0].section_id == "04-1"
+
+
+def test_align_resolver_round_trip_with_legacy_v4_alias():
+    # End-to-end : aligner emits canonical id + alias keys; resolver finds the
+    # legacy decimal key in V4 via alias path (no parent promotion).
+    raw = "### 2.1 First\nbody1\n"
+    sections = [_section("04-2", 2, "2. Parent", raw)]
+    v4 = {"mdx_sections": {"04-2.1": {"judgments_full32": []}}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    sub = out[0]
+    assert sub.section_id == "04-2-sub-1"
+    resolved = _resolve_v4_section_key(
+        v4, sub.section_id, alias_keys=sub.v4_alias_keys
+    )
+    assert resolved == "04-2.1"