feat(joinir): Structural pattern detection + Pattern 4 scaffold

- Add LoopFeatures struct for structure-based detection (no name deps)
- Add LoopPatternKind enum and classify() function
- Pattern 3: has_if_else_phi && !has_break && !has_continue
- Pattern 4: has_continue == true (detection only, lowering TODO)
- Unify router to use extract_features()/classify() instead of legacy
- Remove AST dependency, use LoopForm/LoopScope only
- Add Pattern 4 test file (loop_continue_pattern4.hako)
- Pattern 3 test passes (sum=9)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/patterns/mod.rs

@@ -4,6 +4,7 @@
 //! - Pattern 1: Simple While Loop (pattern1_minimal.rs)
 //! - Pattern 2: Loop with Conditional Break (pattern2_with_break.rs)
 //! - Pattern 3: Loop with If-Else PHI (pattern3_with_if_phi.rs)
+//! - Pattern 4: Loop with Continue (pattern4_with_continue.rs) [Phase 194+]
 //!
 //! Phase 194: Table-driven router for pattern dispatch
 //! - Router module provides table-driven pattern matching
@@ -13,6 +14,7 @@
 pub(in crate::mir::builder) mod pattern1_minimal;
 pub(in crate::mir::builder) mod pattern2_with_break;
 pub(in crate::mir::builder) mod pattern3_with_if_phi;
+pub(in crate::mir::builder) mod pattern4_with_continue;
 pub(in crate::mir::builder) mod router;
 
 // Re-export router for convenience