refactor(joinir): Phase 33-17-B LoopHeaderPhiInfo extraction

## Changes
- Extract LoopHeaderPhiInfo Box (116 lines) from loop_header_phi_builder
- loop_header_phi_builder.rs reduced: 318 → 217 lines (-32%)
- Proper separation of data structures from builder logic

## Module Structure
- loop_header_phi_info.rs: LoopHeaderPhiInfo, CarrierPhiEntry structs + tests
- loop_header_phi_builder.rs: LoopHeaderPhiBuilder implementation

## Box Theory Compliance
- Data structures separated from builder logic
- Each file has clear single responsibility
- Clean re-exports through mod.rs

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

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

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

@@ -17,13 +17,16 @@ mod value_collector;
 mod instruction_rewriter;
 mod exit_phi_builder;
 pub mod exit_line;
-pub mod loop_header_phi_builder;
+mod loop_header_phi_info;
+mod loop_header_phi_builder;
 mod tail_call_classifier;
 mod merge_result;
 
 // Phase 33-17: Re-export for use by other modules
 pub use merge_result::MergeResult;
 pub use tail_call_classifier::{TailCallKind, classify_tail_call};
+pub use loop_header_phi_info::{LoopHeaderPhiInfo, CarrierPhiEntry};
+pub use loop_header_phi_builder::LoopHeaderPhiBuilder;
 
 use crate::mir::{BasicBlockId, MirModule, ValueId};
 use crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary;
@@ -154,7 +157,7 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
                 );
             }
 
-            let phi_info = loop_header_phi_builder::LoopHeaderPhiBuilder::build(
+            let phi_info = LoopHeaderPhiBuilder::build(
                 builder,
                 entry_block_remapped,  // header_block (JoinIR's entry block = loop header)
                 host_entry_block,      // entry_block (host's block that jumps to loop header)
@@ -182,10 +185,10 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
 
             phi_info
         } else {
-            loop_header_phi_builder::LoopHeaderPhiInfo::empty(entry_block_remapped)
+            LoopHeaderPhiInfo::empty(entry_block_remapped)
         }
     } else {
-        loop_header_phi_builder::LoopHeaderPhiInfo::empty(entry_block_remapped)
+        LoopHeaderPhiInfo::empty(entry_block_remapped)
     };
 
     // Phase 4: Merge blocks and rewrite instructions
@@ -212,7 +215,7 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
                 loop_header_phi_info.carrier_phis.len()
             );
         }
-        loop_header_phi_builder::LoopHeaderPhiBuilder::finalize(
+        LoopHeaderPhiBuilder::finalize(
             builder,
             &loop_header_phi_info,
             debug,