feat(joinir): Phase 185-187 body-local infrastructure + string design

Phase 185: Body-local Pattern2/4 integration skeleton
- Added collect_body_local_variables() helper
- Integrated UpdateEnv usage in loop_with_break_minimal
- Test files created (blocked by init lowering)

Phase 186: Body-local init lowering infrastructure
- Created LoopBodyLocalInitLowerer box (378 lines)
- Supports BinOp (+/-/*//) + Const + Variable
- Fail-Fast for method calls/string operations
- 3 unit tests passing

Phase 187: String UpdateLowering design (doc-only)
- Defined UpdateKind whitelist (6 categories)
- StringAppendChar/Literal patterns identified
- 3-layer architecture documented
- No code changes

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

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

src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs

@@ -5,6 +5,38 @@ use crate::mir::builder::MirBuilder;
 use crate::mir::ValueId;
 use super::super::trace;
 
+/// Phase 185-2: Collect body-local variable declarations from loop body
+///
+/// Returns Vec<(name, ValueId)> for variables declared with `local` in loop body.
+/// This function scans the loop body AST for Local nodes and allocates
+/// JoinIR-local ValueIds for each one.
+///
+/// # Arguments
+///
+/// * `body` - Loop body AST nodes to scan
+/// * `alloc_join_value` - JoinIR ValueId allocator function
+///
+/// # Returns
+///
+/// Vector of (variable_name, join_value_id) pairs for all body-local variables
+fn collect_body_local_variables(
+    body: &[ASTNode],
+    alloc_join_value: &mut dyn FnMut() -> ValueId,
+) -> Vec<(String, ValueId)> {
+    let mut locals = Vec::new();
+    for node in body {
+        if let ASTNode::Local { variables, .. } = node {
+            // Local declaration can have multiple variables (e.g., local a, b, c)
+            for name in variables {
+                let value_id = alloc_join_value();
+                locals.push((name.clone(), value_id));
+                eprintln!("[pattern2/body-local] Collected local '{}' → {:?}", name, value_id);
+            }
+        }
+    }
+    locals
+}
+
 /// Phase 194: Detection function for Pattern 2
 ///
 /// Phase 192: Updated to structure-based detection
@@ -140,6 +172,16 @@ impl MirBuilder {
             id
         };
 
+        // Phase 185-2: Collect body-local variables
+        use crate::mir::join_ir::lowering::loop_body_local_env::LoopBodyLocalEnv;
+        let body_locals = collect_body_local_variables(_body, &mut alloc_join_value);
+        let body_local_env = LoopBodyLocalEnv::from_locals(body_locals);
+
+        eprintln!("[pattern2/body-local] Phase 185-2: Collected {} body-local variables", body_local_env.len());
+        for (name, vid) in body_local_env.iter() {
+            eprintln!("  {} → {:?}", name, vid);
+        }
+
         // Debug: Log condition bindings
         eprintln!("[cf_loop/pattern2] Phase 171-172: ConditionEnv contains {} variables:", env.len());
         eprintln!("  Loop param '{}' → JoinIR ValueId(0)", loop_var_name);
@@ -273,6 +315,7 @@ impl MirBuilder {
             &env,
             &carrier_info,
             &carrier_updates,
+            Some(&body_local_env),  // Phase 185-2: Pass body-local environment
         ) {
             Ok((module, meta)) => (module, meta),
             Err(e) => {