refactor(joinir): Pattern 4 modularization with CarrierInfo/ExitMeta

Removes hardcoded "sum" and ValueId(15) from Pattern 4 lowerer by
introducing CarrierInfo and ExitMeta structures.

Changes:
- New carrier_info.rs: CarrierInfo, CarrierVar, ExitMeta structs
- loop_with_continue_minimal.rs: Returns (JoinModule, ExitMeta)
- pattern4_with_continue.rs: Dynamic binding generation from metadata

Design approach: "Thin meta on existing boxes" (ChatGPT proposal)
- CarrierInfo: Built from variable_map, not AST re-analysis
- ExitMeta: Carrier name + JoinIR ValueId pairs from lowerer
- LoopExitBinding: Auto-generated from CarrierInfo + ExitMeta

Test: loop_continue_pattern4.hako outputs 25 (unchanged)

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

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

src/mir/join_ir/lowering/carrier_info.rs

@@ -0,0 +1,64 @@
+//! Carrier variable metadata for JoinIR loop lowering
+//!
+//! This module defines metadata structures for tracking carrier variables
+//! in loop lowering. This enables dynamic generation of exit bindings
+//! without hardcoded variable names or ValueIds.
+
+use crate::mir::ValueId;
+
+/// Information about a single carrier variable
+#[derive(Debug, Clone)]
+pub struct CarrierVar {
+    /// Variable name (e.g., "sum", "printed")
+    pub name: String,
+    /// Host ValueId for this variable
+    pub host_id: ValueId,
+}
+
+/// Complete carrier information for a loop
+#[derive(Debug, Clone)]
+pub struct CarrierInfo {
+    /// Loop control variable name (e.g., "i")
+    pub loop_var_name: String,
+    /// Loop control variable ValueId in host
+    pub loop_var_id: ValueId,
+    /// Additional carrier variables (e.g., sum, printed)
+    pub carriers: Vec<CarrierVar>,
+}
+
+/// Exit metadata returned by lowerers
+///
+/// This structure captures the mapping from JoinIR exit values to
+/// carrier variable names, enabling dynamic binding generation.
+#[derive(Debug, Clone)]
+pub struct ExitMeta {
+    /// Exit value bindings: (carrier_name, join_exit_value_id)
+    ///
+    /// Example for Pattern 4:
+    /// ```
+    /// vec![("sum".to_string(), ValueId(15))]
+    /// ```
+    /// where ValueId(15) is the k_exit parameter in JoinIR-local space.
+    pub exit_values: Vec<(String, ValueId)>,
+}
+
+impl ExitMeta {
+    /// Create new ExitMeta with no exit values
+    pub fn empty() -> Self {
+        Self {
+            exit_values: vec![],
+        }
+    }
+
+    /// Create ExitMeta with a single exit value
+    pub fn single(carrier_name: String, join_value: ValueId) -> Self {
+        Self {
+            exit_values: vec![(carrier_name, join_value)],
+        }
+    }
+
+    /// Create ExitMeta with multiple exit values
+    pub fn multiple(exit_values: Vec<(String, ValueId)>) -> Self {
+        Self { exit_values }
+    }
+}

src/mir/join_ir/lowering/loop_with_continue_minimal.rs

@@ -68,6 +68,7 @@
 //!
 //! Following the "80/20 rule" from CLAUDE.md - get it working first, generalize later.
 
+use crate::mir::join_ir::lowering::carrier_info::ExitMeta;
 use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
 use crate::mir::join_ir::{
     BinOpKind, CompareOp, ConstValue, JoinFuncId, JoinFunction, JoinInst, JoinModule,
@@ -101,7 +102,7 @@ use crate::mir::ValueId;
 ///
 /// # Returns
 ///
-/// * `Some(JoinModule)` - Successfully lowered to JoinIR
+/// * `Some((JoinModule, ExitMeta))` - Successfully lowered to JoinIR with exit metadata
 /// * `None` - Pattern not matched (fallback to other lowerers)
 ///
 /// # Boundary Contract
@@ -109,8 +110,9 @@ use crate::mir::ValueId;
 /// This function returns a JoinModule with:
 /// - **Input slots**: ValueId(0) = i_init, ValueId(1) = sum_init
 /// - **Output slot**: k_exit returns the final sum value
+/// - **Exit metadata**: ExitMeta containing ("sum", ValueId(15)) binding
 /// - **Caller responsibility**: Create JoinInlineBoundary to map ValueIds
-pub fn lower_loop_with_continue_minimal(_scope: LoopScopeShape) -> Option<JoinModule> {
+pub fn lower_loop_with_continue_minimal(_scope: LoopScopeShape) -> Option<(JoinModule, ExitMeta)> {
     // Phase 195: Use local ValueId allocator (sequential from 0)
     // JoinIR has NO knowledge of host ValueIds - boundary handled separately
     let mut value_counter = 0u32;
@@ -345,5 +347,11 @@ pub fn lower_loop_with_continue_minimal(_scope: LoopScopeShape) -> Option<JoinMo
     eprintln!("[joinir/pattern4] Continue: Jump(loop_step) to skip iteration");
     eprintln!("[joinir/pattern4] Carriers: i, sum");
 
-    Some(join_module)
+    // Phase 196: Return ExitMeta with carrier bindings
+    // k_exit parameter sum_exit (ValueId(15)) should be bound to host's "sum" variable
+    let exit_meta = ExitMeta::single("sum".to_string(), sum_exit);
+
+    eprintln!("[joinir/pattern4] ExitMeta: sum → ValueId({})", sum_exit.0);
+
+    Some((join_module, exit_meta))
 }

src/mir/join_ir/lowering/mod.rs

@@ -15,6 +15,7 @@
 //! - `if_select.rs`: Phase 33 If/Else → Select lowering
 //! - `if_dry_runner.rs`: Phase 33-10 If lowering dry-run スキャナー（箱化版）
 
+pub mod carrier_info; // Phase 196: Carrier metadata for loop lowering
 pub mod common;
 pub mod exit_args_resolver;
 pub mod funcscanner_append_defs;