feat(joinir): Phase 53 - SELFHOST-NORM-DEV-EXPAND implementation

Expanded selfhost dev Normalized target with 2 practical P2/P3 loop variations,
strengthened structural signature axis, and implemented two-stage detection.

Key Changes:

1. Documentation (phase49-selfhost-joinir-depth2-design.md +128 lines):
   - Added Phase 53 section with candidate selection rationale
   - Documented two-stage detector strategy (structural primary + dev-only name guard)
   - Defined structural axis strengthening (carrier count/type, branch patterns)

2. Fixtures (+210 lines):
   - selfhost_args_parse_p2.program.json (60 lines): P2 with String carrier + conditional branching
   - selfhost_stmt_count_p3.program.json (150 lines): P3 with 5 carriers + multi-branch if-else

3. Structured Builders (fixtures.rs +48 lines):
   - build_selfhost_args_parse_p2_structured_for_normalized_dev()
   - build_selfhost_stmt_count_p3_structured_for_normalized_dev()

4. ShapeGuard Two-Stage Detection (shape_guard.rs +80 lines):
   - Added SelfhostArgsParseP2/SelfhostStmtCountP3 to NormalizedDevShape enum
   - Implemented is_selfhost_args_parse_p2(): P2 core family + name guard
   - Implemented is_selfhost_stmt_count_p3(): 2-10 carrier check + name guard
   - Updated capability_for_shape() mappings

5. Bridge Integration (bridge.rs +8 lines, normalized.rs +10 lines):
   - Added shape handlers delegating to existing normalizers
   - Added roundtrip reconstruction handlers

6. Entry Point Registration (ast_lowerer/mod.rs +2 lines):
   - Registered selfhost_args_parse_p2/selfhost_stmt_count_p3 as LoopFrontend routes

7. Dev VM Comparison Tests (normalized_joinir_min.rs +40 lines):
   - normalized_selfhost_args_parse_p2_vm_bridge_direct_matches_structured()
   - normalized_selfhost_stmt_count_p3_vm_bridge_direct_matches_structured()

8. Test Context Fix (dev_env.rs):
   - Added thread-local test context depth counter
   - Fixed deadlock in nested test_ctx() calls via reentrant with_dev_env_if_unset()

Structural Axis Growth:

P2 family:
- Carrier count: 1-3 (unchanged)
- NEW: Type diversity (Integer/String mixed)
- NEW: Conditional branching patterns (Eq-heavy comparisons)

P3 family:
- NEW: Carrier count upper bound: 2-10 (was 2-4)
- NEW: Multi-branch if-else (5+ branches with nested structure)
- NEW: Complex conditional patterns

Test Results:
- normalized_dev: 40/40 PASS (including 2 new tests)
- lib regression: 939 PASS, 56 ignored
- Existing behavior unchanged (normalized_dev feature-gated)

Phase 53 Achievements:
✅ P2/P3 each gained 1 practical variation (2 total)
✅ Two-stage detection: structural primary + dev-only name guard
✅ Structural axis expanded: 4 axes (carrier count/type/Compare/branch patterns)
✅ All tests PASS, no regressions
✅ Test context deadlock fixed (0.04s for 29 tests)

Files Modified: 14 files
Lines Added: ~516 lines (net)
Implementation: Pure additive (feature-gated)

Next Phase (54+):
- Accumulate 6+ loops per P2/P3 family
- Achieve 5+ stable structural axes
- Target < 5% false positive rate
- Then shrink/remove name guard scope

src/mir/join_ir_runner.rs

@@ -50,6 +50,31 @@ pub fn run_joinir_function(
     entry: JoinFuncId,
     args: &[JoinValue],
 ) -> Result<JoinValue, JoinRuntimeError> {
+    #[cfg(feature = "normalized_dev")]
+    {
+        // Canonical shapes always go through Normalized roundtrip regardless of mode/env.
+        let canonical_shapes = shape_guard::canonical_shapes(module);
+        if !canonical_shapes.is_empty() {
+            let args_vec = args.to_vec();
+            return dev_env::with_dev_env_if_unset(|| {
+                let structured = normalized_dev_roundtrip_structured(module).map_err(|msg| {
+                    JoinRuntimeError::new(format!(
+                        "[joinir/normalized-dev/runner] canonical roundtrip failed: {}",
+                        msg
+                    ))
+                })?;
+                if dev_env::normalized_dev_logs_enabled() {
+                    eprintln!(
+                        "[joinir/normalized-dev/runner] canonical normalized roundtrip (shapes={:?}, functions={})",
+                        canonical_shapes,
+                        structured.functions.len()
+                    );
+                }
+                execute_function(vm, &structured, entry, args_vec)
+            });
+        }
+    }
+
     #[cfg(feature = "normalized_dev")]
     match current_joinir_mode() {
         JoinIrMode::NormalizedDev => {