Phase A リファクタリング: String.length デバッグ基盤の強化

## 実装内容（振る舞い変更なし）

A-1: Hostcall シンボルの定数化
- 直書き文字列を SYM_* 定数に統一
- nyash.handle.of / nyash.string.len_h / nyash.console.birth_h

A-2: string_len ヘルパー抽出（共通化）
- src/jit/lower/core/string_len.rs 新設
- emit_len_with_fallback_{param,local_handle,literal} を移設
- 二段フォールバック（string.len_h → any.length_h）の集約

A-3: 観測の統一
- import 呼び出しトレース機能を追加（NYASH_JIT_TRACE_IMPORT=1）
- CraneliftBuilder/ObjectBuilder の emit_host_call に構造化イベント
- observe::lower_hostcall で len_h/any.length_h の追跡

## 今後の道筋（CURRENT_TASK.md に記載）
- P0: フェイルセーフ（NYASH_LEN_FORCE_BRIDGE=1）
- P1: シンボル解決の可視化
- P2: リテラル最優先の安定化
- P3: Return 材化の後方走査

バグは手強いけど、デバッグ基盤が整ったにゃ！

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

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

src/jit/lower/builder/object.rs

@@ -285,6 +285,22 @@ impl IRBuilder for ObjectBuilder {
     fn emit_host_call(&mut self, symbol: &str, argc: usize, has_ret: bool) {
         use cranelift_codegen::ir::{AbiParam, Signature, types};
         use cranelift_frontend::FunctionBuilder;
+        // Structured lower event for import call (AOT builder)
+        {
+            let mut arg_types: Vec<&'static str> = Vec::new();
+            for _ in 0..argc { arg_types.push("I64"); }
+            crate::jit::events::emit_lower(
+                serde_json::json!({
+                    "id": symbol,
+                    "decision": "allow",
+                    "reason": "import_call",
+                    "argc": argc,
+                    "arg_types": arg_types,
+                    "ret": if has_ret { "I64" } else { "Void" }
+                }),
+                "hostcall","<aot>"
+            );
+        }
         let mut fb = FunctionBuilder::new(&mut self.ctx.func, &mut self.fbc);
         if let Some(idx) = self.current_block_index { fb.switch_to_block(self.blocks[idx]); }
         else if let Some(b) = self.entry_block { fb.switch_to_block(b); }
@@ -304,6 +320,22 @@ impl IRBuilder for ObjectBuilder {
     fn emit_host_call_typed(&mut self, symbol: &str, params: &[super::ParamKind], has_ret: bool, ret_is_f64: bool) {
         use cranelift_codegen::ir::{AbiParam, Signature, types};
         use cranelift_frontend::FunctionBuilder;
+        // Structured lower event for typed import call (AOT builder)
+        {
+            let mut arg_types: Vec<&'static str> = Vec::new();
+            for k in params { arg_types.push(match k { super::ParamKind::I64 | super::ParamKind::B1 => "I64", super::ParamKind::F64 => "F64" }); }
+            crate::jit::events::emit_lower(
+                serde_json::json!({
+                    "id": symbol,
+                    "decision": "allow",
+                    "reason": "import_call_typed",
+                    "argc": params.len(),
+                    "arg_types": arg_types,
+                    "ret": if has_ret { if ret_is_f64 { "F64" } else { "I64" } } else { "Void" }
+                }),
+                "hostcall","<aot>"
+            );
+        }
         let mut fb = FunctionBuilder::new(&mut self.ctx.func, &mut self.fbc);
         if let Some(idx) = self.current_block_index { fb.switch_to_block(self.blocks[idx]); }
         else if let Some(b) = self.entry_block { fb.switch_to_block(b); }