feat(joinir): Phase 259 P0 - Pattern8 BoolPredicateScan + Copy binding fix

Pattern8 (Boolean Predicate Scan) implementation for is_integer/1:
- New pattern detection for `loop + if not predicate() { return false }`
- JoinIR lowerer with main/loop_step/k_exit structure
- Me receiver passed as param (by-name 禁止)

Key fixes:
1. expr_result = Some(join_exit_value) (Pattern7 style)
2. Tail-call: dst: None (no extra Ret instruction)
3. instruction_rewriter: Add `&& is_loop_header_with_phi` check
   - Pattern8 has no carriers → no PHIs → MUST generate Copy bindings
   - Without this, ValueId(103/104/105) were undefined

Status: Copy instructions now generated correctly, but exit block
creation issue remains (next step: Step A-C in指示書).

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

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

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

@@ -203,6 +203,11 @@ pub(crate) static LOOP_PATTERNS: &[LoopPatternEntry] = &[
         detect: super::pattern7_split_scan::can_lower,
         lower: super::pattern7_split_scan::lower,
     },
+    LoopPatternEntry {
+        name: "Pattern8_BoolPredicateScan", // Phase 259 P0: boolean predicate scan (is_integer/is_valid)
+        detect: super::pattern8_scan_bool_predicate::can_lower,
+        lower: super::pattern8_scan_bool_predicate::lower,
+    },
     LoopPatternEntry {
         name: "Pattern3_WithIfPhi",
         detect: super::pattern3_with_if_phi::can_lower,