refactor(joinir): unify boundary join_inputs SSOT (pattern4/6/7)

Apply Phase 256.8 SSOT fix to Pattern4/6/7:
- Use join_module.entry.params.clone() instead of hardcoded ValueIds
- Add fail-fast validation for params count mismatch
- Remove ValueId(0), ValueId(PARAM_MIN + k) patterns
- Clean up unused PARAM_MIN imports

This prevents entry_param_mismatch errors structurally and maintains
consistency with Pattern2/3.

Changes:
- pattern4_with_continue.rs: Lines 442-476 (SSOT extraction + validation)
- pattern6_scan_with_init.rs: Lines 447-471 (SSOT extraction + validation)
- pattern7_split_scan.rs: Lines 495-526 (SSOT extraction + validation)

All patterns now use the same SSOT principle:
1. Extract entry function (priority: join_module.entry → fallback "main")
2. Use params as SSOT: join_inputs = entry_func.params.clone()
3. Build host_inputs in expected order (pattern-specific)
4. Fail-fast validation: join_inputs.len() == host_inputs.len()

Verification:
- cargo build --release: ✅ PASS (no PARAM_MIN warnings)
- Quick profile: ✅ First FAIL still json_lint_vm (baseline maintained)
- Pattern6 smoke: ✅ PASS (index_of test)
- Pattern7 smoke: Pre-existing phi pred mismatch (not introduced by SSOT)

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

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

src/mir/join_ir/lowering/expr_lowerer.rs

@@ -15,7 +15,7 @@
 //! **Fail-Safe**: Unsupported AST nodes return explicit errors, allowing callers
 //! to fall back to legacy paths.
 
-use super::condition_lowerer::lower_condition_to_joinir_no_body_locals;
+use super::condition_lowerer::{lower_condition_to_joinir, lower_condition_to_joinir_no_body_locals};
 use super::scope_manager::ScopeManager;
 use crate::ast::ASTNode;
 use crate::mir::builder::MirBuilder;
@@ -296,9 +296,15 @@ impl<'env, 'builder, S: ScopeManager> ConditionLoweringBox<S> for ExprLowerer<'e
                 .map_err(|e| e.to_string())?;
 
         // Delegate to the well-tested lowerer, but use the caller-provided allocator (SSOT).
-        let (result_value, instructions) =
-            lower_condition_to_joinir_no_body_locals(condition, &mut *context.alloc_value, &condition_env) // Phase 92 P2-2
-                .map_err(|e| e.to_string())?;
+        // Phase 256.7: Pass current_static_box_name for this.method(...) support
+        let (result_value, instructions) = lower_condition_to_joinir(
+            condition,
+            &mut *context.alloc_value,
+            &condition_env,
+            None, // body_local_env
+            context.current_static_box_name.as_deref(), // Phase 256.7
+        )
+        .map_err(|e| e.to_string())?;
 
         self.last_instructions = instructions;