feat(joinir): Phase 194 - Table-driven loop pattern router

Replace if/else chain with table-driven pattern dispatch for easier
pattern addition and maintenance.

# Changes

## New Files
- router.rs (137 lines): Pattern router table and dispatch logic
  - LoopPatternContext: Context passed to detect/lower functions
  - LoopPatternEntry: Pattern registration structure
  - LOOP_PATTERNS: Static table with 3 registered patterns
  - route_loop_pattern(): Single dispatch function

## Modified Files
- patterns/mod.rs (+8 lines): Export router module
- pattern1_minimal.rs (+19 lines): Added can_lower() + lower() wrapper
- pattern2_with_break.rs (+17 lines): Added can_lower() + lower() wrapper
- pattern3_with_if_phi.rs (+22 lines): Added can_lower() + lower() wrapper
- routing.rs (-21 lines): Replaced if/else chain with router call

# Architecture Improvement

## Before (if/else chain)
```rust
if func_name == "main" && has_sum {
    return pattern3(...);
} else if func_name == "main" {
    return pattern1(...);
} else if func_name == "JoinIrMin.main/0" {
    return pattern2(...);
}
```

## After (table-driven)
```rust
let ctx = LoopPatternContext::new(...);
route_loop_pattern(self, &ctx)?
```

# Adding New Patterns (Now Trivial!)

1. Create pattern4_your_name.rs
2. Implement can_lower() + lower()
3. Add entry to LOOP_PATTERNS table

That's it! No routing logic changes needed.

# Testing

✅ Pattern 1 (loop_min_while.hako): PASSED
✅ Pattern 2 (joinir_min_loop.hako): PASSED
✅ Pattern 3 (loop_if_phi.hako): Routes correctly (VM error is pre-existing)

Router logging verified:
```
NYASH_TRACE_VARMAP=1 ./target/release/hakorune apps/tests/*.hako
[route] Pattern 'Pattern1_Minimal' matched for function 'main'
[route] Pattern 'Pattern2_WithBreak' matched for function 'JoinIrMin.main/0'
[route] Pattern 'Pattern3_WithIfPhi' matched for function 'main'
```

# Line Counts

- router.rs: 137 lines (new)
- Total pattern files: 491 lines (3 patterns)
- routing.rs: Reduced by 21 lines (-6%)
- Net addition: +137 lines (infrastructure investment)

# Documentation

See router.rs header for:
- Architecture overview
- How to add new patterns
- Priority ordering (Pattern3=30, Pattern1=10, Pattern2=20)

Phase 194 complete - pattern addition is now a trivial task!

🤖 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

@@ -4,7 +4,16 @@
 //! - 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)
+//!
+//! Phase 194: Table-driven router for pattern dispatch
+//! - Router module provides table-driven pattern matching
+//! - Each pattern exports can_lower() and lower() functions
+//! - See router.rs for how to add new patterns
 
 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 router;
+
+// Re-export router for convenience
+pub(in crate::mir::builder) use router::{route_loop_pattern, LoopPatternContext};