Phase 33-4: Stage-1/Stage-B expansion complete (33-4.1 to 33-4.3)

## Code Changes
- Extended function name whitelist in try_lower_if_to_joinir()
- Added: JsonShapeParser._read_value_from_pair/1 (Stage-1)
- Added: Stage1JsonScannerBox.value_start_after_key_pos/2 (Stage-B)

## Validation
- A/B testing: Route A (if_phi) vs Route B (Select) → identical results (RC 0)
- Test cases: joinir_if_select_simple.hako, joinir_if_select_local.hako
- Build: cargo build --release successful

## Documentation (docs/private submodule)
- TASKS.md: Phase 33-4.1 to 33-4.3 marked complete
- if_joinir_design.md: Section 9 added (candidate analysis)

## Next Steps
- Phase 33-4.4: CI/smoke test updates (pending)

src/mir/join_ir/lowering/mod.rs

@@ -49,7 +49,10 @@ use crate::mir::{BasicBlockId, MirFunction};
 /// Phase 33-3: Try to lower if/else to JoinIR Select instruction
 ///
 /// Scope:
-/// - Only applies to functions matching "IfSelectTest.*"
+/// - Only applies to whitelisted functions:
+///   - IfSelectTest.* (Phase 33-2/33-3)
+///   - JsonShapeParser._read_value_from_pair/1 (Phase 33-4 Stage-1)
+///   - Stage1JsonScannerBox.value_start_after_key_pos/2 (Phase 33-4 Stage-B)
 /// - Requires NYASH_JOINIR_IF_SELECT=1 environment variable
 /// - Falls back to traditional if_phi on pattern mismatch
 ///
@@ -64,11 +67,15 @@ pub fn try_lower_if_to_joinir(
         return None;
     }
 
-    // 関数名で制限（IfSelectTest.* のみ）
-    if !func.signature.name.starts_with("IfSelectTest.") {
+    // Phase 33-4: 関数名ガード拡張（IfSelectTest + Stage-1/Stage-B 候補）
+    let is_allowed = func.signature.name.starts_with("IfSelectTest.")
+        || func.signature.name == "JsonShapeParser._read_value_from_pair/1"
+        || func.signature.name == "Stage1JsonScannerBox.value_start_after_key_pos/2";
+
+    if !is_allowed {
         if debug {
             eprintln!(
-                "[try_lower_if_to_joinir] skipping non-test function: {}",
+                "[try_lower_if_to_joinir] skipping non-allowed function: {}",
                 func.signature.name
             );
         }