fix(mir): SSA違反修正 & StringBox is_space/starts_with実装

Task A: ローカル変数SSA違反修正
- src/mir/builder/stmts.rs: Copy命令で一意ValueId割り当て
- 元のエラー "Invalid value: use of undefined value" 解決
- using_resolver_box.hako が正常動作確認

Task B: StringBox新メソッド実装
- plugins/nyash-string-plugin: is_space/starts_with追加
- M_IS_SPACE (7), M_STARTS_WITH (8) 実装
- string_helpers.hako仕様に準拠

残存問題: do_break()のunreachableブロック生成
→ 次のコミットで修正予定

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

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

plugins/nyash-string-plugin/src/lib.rs

@@ -25,6 +25,8 @@ const M_CHAR_CODE_AT: u32 = 3;
 const M_CONCAT: u32 = 4; // concat(other: String|Handle) -> Handle(new)
 const M_FROM_UTF8: u32 = 5; // fromUtf8(data: String|Bytes) -> Handle(new)
 const M_TO_UTF8: u32 = 6; // toUtf8() -> String
+const M_IS_SPACE: u32 = 7; // is_space(ch: String) -> bool
+const M_STARTS_WITH: u32 = 8; // starts_with(src: String, i: i64, pat: String) -> bool
 const M_FINI: u32 = u32::MAX;
 
 const TYPE_ID_STRING: u32 = 10; // Match nyash.toml type_id
@@ -223,6 +225,8 @@ extern "C" fn string_resolve(name: *const c_char) -> u32 {
         "concat" => M_CONCAT,
         "fromUtf8" => M_FROM_UTF8,
         "toUtf8" | "toString" => M_TO_UTF8, // Map toString to toUtf8
+        "is_space" => M_IS_SPACE,
+        "starts_with" => M_STARTS_WITH,
         _ => 0,
     }
 }
@@ -331,6 +335,52 @@ extern "C" fn string_invoke_id(
                     return E_PLUGIN;
                 }
             }
+            M_IS_SPACE => {
+                // is_space(ch: String) -> bool
+                // Check if single character is whitespace: " ", "\t", "\n", "\r"
+                let ch = match read_arg_string(args, args_len, 0) {
+                    Some(s) => s,
+                    None => return E_ARGS,
+                };
+                let is_space = ch == " " || ch == "\t" || ch == "\n" || ch == "\r";
+                return write_tlv_bool(is_space, result, result_len);
+            }
+            M_STARTS_WITH => {
+                // starts_with(src: String, i: i64, pat: String) -> bool
+                // Check if 'src' starts with 'pat' at position 'i'
+                // Args: [0] = src (String), [1] = i (i64), [2] = pat (String)
+                let src = match read_arg_string(args, args_len, 0) {
+                    Some(s) => s,
+                    None => return E_ARGS,
+                };
+                let i = match read_arg_i64(args, args_len, 1) {
+                    Some(v) if v >= 0 => v as usize,
+                    _ => return E_ARGS,
+                };
+                let pat = match read_arg_string(args, args_len, 2) {
+                    Some(s) => s,
+                    None => return E_ARGS,
+                };
+
+                let src_len = src.len();
+                let pat_len = pat.len();
+
+                // Check bounds: i + pat.length() > src.length() → false
+                if i + pat_len > src_len {
+                    return write_tlv_bool(false, result, result_len);
+                }
+
+                // Character-by-character comparison
+                let src_bytes = src.as_bytes();
+                let pat_bytes = pat.as_bytes();
+                for k in 0..pat_len {
+                    if src_bytes[i + k] != pat_bytes[k] {
+                        return write_tlv_bool(false, result, result_len);
+                    }
+                }
+
+                return write_tlv_bool(true, result, result_len);
+            }
             _ => E_METHOD,
         }
     }