fix(joinir): Phase 287 P2 - Pattern6 nested loop latch overwrite fix

Fix infinite loop in Pattern6 (nested loop minimal) caused by main→loop_step
overwriting k_inner_exit→loop_step latch values.

Root cause: JoinIR main entry block was incorrectly treated as BackEdge,
causing it to overwrite the correct latch incoming values set by the true
back edge (k_inner_exit → loop_step).

Solution:
- Restrict latch recording to TailCallKind::BackEdge only
- Treat only MAIN's entry block as entry-like (not loop_step's entry block)
- Add debug_assert! to detect double latch set in future

Refactoring:
- Extract latch recording to latch_incoming_recorder module (SSOT)
- Add boundary.loop_header_func_name for explicit header identification
- Strengthen tail_call_classifier with is_source_entry_like parameter

Tests: apps/tests/phase1883_nested_minimal.hako → RC:9 (was infinite loop)
Smoke: 154/154 PASS, no regressions

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

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

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

@@ -383,6 +383,7 @@ mod tests {
             expr_result: None,          // Phase 33-14: Add missing field
             jump_args_layout: crate::mir::join_ir::lowering::inline_boundary::JumpArgsLayout::CarriersOnly,
             loop_var_name: None,        // Phase 33-16: Add missing field
+            loop_header_func_name: None, // Phase 287 P2
             carrier_info: None,         // Phase 228: Add missing field
             loop_invariants: vec![],    // Phase 255 P2: Add missing field
             continuation_func_ids: std::collections::BTreeSet::from([

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

@@ -137,6 +137,7 @@ mod tests {
             expr_result: None,          // Phase 33-14: Add missing field
             jump_args_layout: crate::mir::join_ir::lowering::inline_boundary::JumpArgsLayout::CarriersOnly,
             loop_var_name: None,        // Phase 33-16: Add missing field
+            loop_header_func_name: None, // Phase 287 P2
             carrier_info: None,         // Phase 228: Add missing field
             loop_invariants: vec![],    // Phase 255 P2: Add missing field
             continuation_func_ids: std::collections::BTreeSet::from([