feat(joinir): Phase 46 - P2-Mid canonical Normalized promotion

Promote P2-Mid patterns (_atoi real, _parse_number real) to canonical
Normalized→MIR(direct) route, completing P2 line transition.

Canonical set expansion (Phase 41 → Phase 46):
- P2-Core: Pattern2Mini, skip_ws mini/real, atoi mini
- P2-Mid: atoi real, parse_number real (NEW)

All JsonParser P2 loops (_skip_whitespace, _atoi, _parse_number) now
canonical Normalized - Structured→MIR is legacy/comparison-only.

Key changes:
- shape_guard.rs: Expanded is_canonical_shape() (+2 patterns)
  - JsonparserAtoiReal
  - JsonparserParseNumberReal
  - Made NormalizedDevShape enum public
- bridge.rs: Updated canonical routing comments (Phase 41 → 46)
- normalized.rs: Made shape_guard module public
- normalized_joinir_min.rs: Added Phase 46 canonical verification test
- phase46-norm-canon-p2-mid.md: Complete design documentation

Out of scope (deferred):
- P3/P4 Normalized support → NORM-P3/NORM-P4 phases
- Selfhost complex loops → separate phases

Benefits:
- Clear P2 boundary: All JsonParser P2 = Normalized canonical
- Infrastructure validation: Phase 43/245B proven production-ready
- Simplified mental model: P2 = Normalized-first, P3/P4 = future

Tests: 937/937 PASS (lib), 20/20 PASS (normalized_dev feature)
Phase 46 test: test_phase46_canonical_set_includes_p2_mid ✅

src/mir/join_ir/normalized/shape_guard.rs

@@ -19,7 +19,7 @@ pub enum ShapeCapabilityKind {
     /// P2 Mid: _parse_number real (p + num_str + result)
     P2MidParseNumber,
 
-    /// Future: Other P2 patterns
+    // Future: Other P2 patterns
     // P2MidAtOfLoop,
     // P2HeavyString,
 }
@@ -42,7 +42,7 @@ impl ShapeCapability {
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub(crate) enum NormalizedDevShape {
+pub enum NormalizedDevShape {
     Pattern1Mini,
     Pattern2Mini,
     JsonparserSkipWsMini,
@@ -109,15 +109,25 @@ pub fn capability_for_shape(shape: &NormalizedDevShape) -> ShapeCapability {
     ShapeCapability::new(kind)
 }
 
-/// Phase 44: Check if shape is canonical P2-Core (shape-level check)
+/// Phase 46: Canonical shapes that ALWAYS use Normalized→MIR(direct)
+/// regardless of feature flags or mode.
 ///
-/// Phase 41 canonical set (exact): Pattern2Mini, skip_ws (mini/real), atoi mini.
-/// Excludes: Pattern1Mini, atoi real, parse_number real.
+/// Canonical set (Phase 46):
+/// - P2-Core: Pattern2Mini, JsonparserSkipWsMini, JsonparserSkipWsReal, JsonparserAtoiMini
+/// - P2-Mid: JsonparserAtoiReal, JsonparserParseNumberReal
+///
+/// P3/P4 patterns are NOT canonical (deferred to NORM-P3/NORM-P4 phases).
 pub fn is_canonical_shape(shape: &NormalizedDevShape) -> bool {
     use NormalizedDevShape::*;
     matches!(
         shape,
-        Pattern2Mini | JsonparserSkipWsMini | JsonparserSkipWsReal | JsonparserAtoiMini
+        Pattern2Mini
+            | JsonparserSkipWsMini
+            | JsonparserSkipWsReal
+            | JsonparserAtoiMini
+            // Phase 46: Add P2-Mid patterns
+            | JsonparserAtoiReal
+            | JsonparserParseNumberReal
     )
 }
 
@@ -137,8 +147,13 @@ pub fn is_supported_by_normalized(cap: &ShapeCapability) -> bool {
 }
 
 /// canonical（常時 Normalized 経路を通す）対象。
-/// Phase 41: P2 コアセット（P2 mini + JP skip_ws mini/real + JP atoi mini）。
-/// Phase 44: Capability-based filtering (backward compatible)。
+/// Phase 46: Extract canonical shapes from JoinModule.
+///
+/// Canonical set (P2-Core + P2-Mid):
+/// - Pattern2Mini, skip_ws mini/real, atoi mini/real, parse_number real
+///
+/// These shapes ALWAYS use Normalized→MIR(direct) regardless of mode.
+/// P3/P4 patterns are NOT canonical (future NORM-P3/NORM-P4 phases).
 pub(crate) fn canonical_shapes(module: &JoinModule) -> Vec<NormalizedDevShape> {
     let shapes: Vec<_> = detect_shapes(module)
         .into_iter()