chore(joinir): Phase 203-A remove dead code after JoinValueSpace unification

Remove obsolete v1 API and unused imports after Phase 201-202 JoinValueSpace unification.

Changes:
- Remove v1 API build_for_break_condition() (70 lines)
  - Converted 3 unit tests to v2 API (build_for_break_condition_v2)
  - Updated module documentation to reflect v2 as standard
- Remove 2 unused imports (JoinValueSpace, ValueId)
  - inline_boundary_builder.rs: Remove unused JoinValueSpace import
  - loop_with_if_phi_minimal.rs: Remove unused ValueId import
- Document stub function (lower_loop_with_break_to_joinir)
  - Add Phase 203-A documentation clarifying stub status
  - Explain why stub exists and migration options
  - Kept stub as it's called by router (line 148 in loop_pattern_router.rs)

Stats:
- 4 files changed
- 75 net lines removed (117 deletions, 42 insertions)
- All tests passing (821 passed)
- Build clean with no errors

Related:
- Phase 201: JoinValueSpace unified ValueId allocation
- Phase 202: Pattern 2 conversion to v2 API

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/join_ir/lowering/loop_patterns/with_break.rs

@@ -86,44 +86,30 @@ pub fn lower_loop_with_break_to_joinir(
     _loop_form: &LoopForm,
     _lowerer: &mut LoopToJoinLowerer,
 ) -> Option<JoinInst> {
-    // Phase 188-Impl-2: Delegate to minimal lowerer
-    // TODO: Extract LoopScopeShape from loop_form for generic implementation
-    use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
-
-    use crate::mir::BasicBlockId;
-    use std::collections::{BTreeMap, BTreeSet};
-
-    // For now, use a placeholder LoopScopeShape
-    // TODO: Build actual LoopScopeShape from loop_form
-    let _placeholder_scope = LoopScopeShape {
-        header: BasicBlockId(0),
-        body: BasicBlockId(0),
-        latch: BasicBlockId(0),
-        exit: BasicBlockId(0),
-        pinned: BTreeSet::new(),
-        carriers: BTreeSet::new(),
-        body_locals: BTreeSet::new(),
-        exit_live: BTreeSet::new(),
-        progress_carrier: None,
-        variable_definitions: BTreeMap::new(),
-    };
-
-    // Generate JoinIR module
-    // Phase 169: lower_loop_with_break_minimal now requires condition AST and builder
-    // This test stub is not used by the actual router, so commenting out for now
-    // let _join_module = lower_loop_with_break_minimal(placeholder_scope, condition, builder)?;
-
-    // Phase 188-Impl-2: Pattern 2 is now integrated in control_flow.rs via cf_loop_pattern2_with_break()
-    // This function (lower_loop_with_break_to_joinir) is currently not used by the router.
-    // The actual lowering happens directly in control_flow.rs which calls loop_with_break_minimal.
+    // Phase 203-A: STUB FUNCTION - Called by router but always returns None
     //
-    // TODO: Either:
-    // 1. Remove this function and rely only on control_flow.rs integration, OR
-    // 2. Implement JoinModule → JoinInst conversion here (future phase)
+    // Status: This function is called by loop_pattern_router.rs:148 but is a NO-OP stub.
+    // The actual Pattern 2 lowering happens via control_flow.rs.
+    //
+    // Why this stub exists:
+    // - Router expects unified interface: lower_*_to_joinir(loop_form, lowerer)
+    // - Pattern 2 is tightly integrated with control_flow.rs
+    // - Removing it would require updating router dispatch logic
+    //
+    // Current behavior:
+    // 1. Router calls this function (line 148 in loop_pattern_router.rs)
+    // 2. Function logs a message and returns None
+    // 3. Router falls back to control_flow.rs hardcoded Pattern 2 route
+    //
+    // Migration options (future):
+    // - Option 1: Remove stub and update router to call control_flow.rs directly
+    // - Option 2: Implement JoinModule → JoinInst conversion here
+    //
+    // Related code:
+    // - Router callsite: loop_pattern_router.rs:148
+    // - Actual implementation: control_flow::cf_loop_pattern2_with_break()
+    // - Minimal lowerer: loop_with_break_minimal::lower_loop_with_break_minimal()
 
-    eprintln!("[loop_patterns] Pattern 2: Lowering delegated to control_flow.rs");
-
-    // Temporary: Return None to trigger fallback
-    // Pattern 2 currently works via hardcoded route in control_flow.rs
+    eprintln!("[loop_patterns] Pattern 2: Lowering delegated to control_flow.rs (stub)");
     None
 }