fix(mir/exit_phi): Pass branch_source_block to build_exit_phis()

## Problem
Exit PHI generation was using header_id as predecessor, but when
build_expression(condition) creates new blocks, the actual branch
instruction is emitted from a different block, causing:
"phi pred mismatch: no input for predecessor BasicBlockId(X)"

## Solution
- Modified build_exit_phis() to accept branch_source_block parameter
- Capture actual block after condition evaluation in loop_builder.rs
- Use branch_source_block instead of header_id for PHI inputs

## Progress
- Error changed from ValueId(5941)/BasicBlockId(4674) to
  ValueId(5927)/BasicBlockId(4672), showing partial fix
- Added comprehensive test suite in mir_loopform_exit_phi.rs
- Added debug logging to trace condition block creation

## Status
Partial fix - unit tests pass, but Test 2 (Stage-B compilation) still
has errors. Needs further investigation of complex nested compilation
scenarios.

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

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

src/tests/mir_loopform_exit_phi.rs

@@ -0,0 +1,174 @@
+/*!
+ * Unit tests for LoopForm v2 exit PHI generation
+ *
+ * Tests the build_exit_phis() implementation in loopform_builder.rs
+ * Focus: predecessor tracking and PHI input generation for break statements
+ */
+
+use crate::parser::NyashParser;
+use crate::mir::{MirCompiler, MirVerifier};
+
+#[test]
+fn test_loopform_exit_phi_single_break() {
+    // Enable LoopForm PHI v2 and MIR verification
+    std::env::set_var("NYASH_LOOPFORM_PHI_V2", "1");
+    std::env::set_var("NYASH_VM_VERIFY_MIR", "1");
+    std::env::set_var("NYASH_LOOPFORM_DEBUG", "1");
+    std::env::set_var("NYASH_PARSER_STAGE3", "1");
+    std::env::set_var("NYASH_PARSER_ALLOW_SEMICOLON", "1");
+
+    let src = r#"
+static box TestExitPhi {
+  test() {
+    local i = 0
+    loop(i < 10) {
+      if i == 5 { break }
+      i = i + 1
+    }
+    return i
+  }
+}
+"#;
+
+    println!("=== Test: Single break statement ===");
+
+    // Parse
+    let ast = NyashParser::parse_from_string(src)
+        .expect("parse failed");
+
+    // Compile
+    let mut mc = MirCompiler::with_options(false);
+    let cr = mc.compile(ast).expect("compile failed");
+
+    // MIR verification
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&cr.module) {
+        for err in &errors {
+            eprintln!("❌ MIR verification error: {}", err);
+        }
+        panic!("❌ MIR verification failed with {} errors", errors.len());
+    }
+    println!("✅ MIR verification passed");
+}
+
+#[test]
+fn test_loopform_exit_phi_multiple_breaks() {
+    std::env::set_var("NYASH_LOOPFORM_PHI_V2", "1");
+    std::env::set_var("NYASH_VM_VERIFY_MIR", "1");
+    std::env::set_var("NYASH_LOOPFORM_DEBUG", "1");
+    std::env::set_var("NYASH_PARSER_STAGE3", "1");
+    std::env::set_var("NYASH_PARSER_ALLOW_SEMICOLON", "1");
+
+    let src = r#"
+static box TestMultiBreak {
+  test() {
+    local i = 0
+    loop(i < 10) {
+      if i == 3 { break }
+      if i == 5 { break }
+      i = i + 1
+    }
+    return i
+  }
+}
+"#;
+
+    println!("=== Test: Multiple break statements ===");
+
+    let ast = NyashParser::parse_from_string(src).expect("parse failed");
+    let mut mc = MirCompiler::with_options(false);
+    let cr = mc.compile(ast).expect("compile failed");
+
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&cr.module) {
+        for err in &errors {
+            eprintln!("❌ MIR verification error: {}", err);
+        }
+        panic!("❌ MIR verification failed with {} errors", errors.len());
+    }
+    println!("✅ MIR verification passed");
+}
+
+#[test]
+fn test_loopform_exit_phi_nested_if_break() {
+    std::env::set_var("NYASH_LOOPFORM_PHI_V2", "1");
+    std::env::set_var("NYASH_VM_VERIFY_MIR", "1");
+    std::env::set_var("NYASH_LOOPFORM_DEBUG", "1");
+    std::env::set_var("NYASH_PARSER_STAGE3", "1");
+    std::env::set_var("NYASH_PARSER_ALLOW_SEMICOLON", "1");
+
+    let src = r#"
+static box TestNestedBreak {
+  test() {
+    local i = 0
+    local found = 0
+    loop(i < 10) {
+      if i > 5 {
+        if i == 7 {
+          found = 1
+          break
+        }
+      }
+      i = i + 1
+    }
+    return found
+  }
+}
+"#;
+
+    println!("=== Test: Nested if with break ===");
+
+    let ast = NyashParser::parse_from_string(src).expect("parse failed");
+    let mut mc = MirCompiler::with_options(false);
+    let cr = mc.compile(ast).expect("compile failed");
+
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&cr.module) {
+        for err in &errors {
+            eprintln!("❌ MIR verification error: {}", err);
+        }
+        panic!("❌ MIR verification failed with {} errors", errors.len());
+    }
+    println!("✅ MIR verification passed");
+}
+
+#[test]
+fn test_loopform_exit_phi_multiple_vars() {
+    std::env::set_var("NYASH_LOOPFORM_PHI_V2", "1");
+    std::env::set_var("NYASH_VM_VERIFY_MIR", "1");
+    std::env::set_var("NYASH_LOOPFORM_DEBUG", "1");
+    std::env::set_var("NYASH_PARSER_STAGE3", "1");
+    std::env::set_var("NYASH_PARSER_ALLOW_SEMICOLON", "1");
+
+    let src = r#"
+static box TestMultiVars {
+  test() {
+    local i = 0
+    local sum = 0
+    local product = 1
+    loop(i < 10) {
+      if sum > 20 { break }
+      sum = sum + i
+      product = product * 2
+      i = i + 1
+    }
+    return sum
+  }
+}
+"#;
+
+    println!("=== Test: Multiple variables with break ===");
+
+    let ast = NyashParser::parse_from_string(src).expect("parse failed");
+    let mut mc = MirCompiler::with_options(false);
+    let cr = mc.compile(ast).expect("compile failed");
+
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&cr.module) {
+        for err in &errors {
+            eprintln!("❌ MIR verification error: {}", err);
+        }
+        panic!("❌ MIR verification failed with {} errors", errors.len());
+    }
+    println!("✅ MIR verification passed");
+}