refactor(engine): 将 engine 从 submodule 内嵌为仓内一等组件

`vendor/MetasequoiaImeEngine` submodule 改为 `engine/` 目录，导入的是
product-lock.json 原本钉住的提交 c810d201，逐文件核对过 blob，因此这是一次
搬迁而不是引擎升级。引擎自己的嵌套 submodule（googlepinyinime-rev、utfcpp、
miniaudio）按各自 gitlink 一并展开，来源与提交见 engine/UPSTREAM.md。

Windows 专门化裁掉了不属于本产品的部分：whisper.cpp 与本地推理 provider、
macOS 示例宿主、整个 dictionary/ 建库流水线、上游 CI 与 release-please 配置。
词库仍按 product-lock.json 在构建时取发布产物。

引擎不再进 product-lock.json：没有 gitlink 就没有可比对的东西，锁里写一个上游
commit 等于宣称这棵树仍等于上游，而第一个 Windows 专门化改动落地时这句话就假了。
verify-contracts / verify-checkout 随之移除，verify-published 保留。

CI 与工具链：引擎源码并入根 scripts/format.sh（删掉引擎自己的那份，避免两套排除
列表分叉）；引擎 voice.yml 里的 silero_vad.onnx 摘要校验移到本仓 lock job；
engine-bump-triage 工作流与其测试删除；gitsubmodule 的 dependabot 频率由 daily
改为 monthly。

README、README.en、AGENTS.md、CONTRIBUTING、docs/product-release.md 及各子目录
文档同步说明新的位置与边界。

engine/helpcode/scripts/filter_zrm_helpcode.py

@@ -0,0 +1,61 @@
+"""筛选自然码辅助码文件中辅助码恰好为两位字符的记录。"""
+
+from __future__ import annotations
+
+import argparse
+from pathlib import Path
+
+
+DEFAULT_INPUT = Path(
+    r"C:\Users\SonnyCalcr\Downloads\ZRM_Aux-code-main\ZRM_Aux-code_4.3.txt"
+)
+DEFAULT_OUTPUT = Path(__file__).resolve().parent.parent / "helpcodes" / "zrm_helpcode.txt"
+
+
+def filter_helpcode(source: Path, destination: Path) -> tuple[int, int]:
+    """保留 `字=两位辅助码` 记录，按完整记录去重并维持原顺序。"""
+    seen: set[str] = set()
+    result: list[str] = []
+    matched_count = 0
+
+    with source.open("r", encoding="utf-8-sig") as input_file:
+        for raw_line in input_file:
+            line = raw_line.rstrip("\r\n")
+            if "=" not in line:
+                continue
+
+            _, helpcode = line.split("=", 1)
+            if len(helpcode) != 2:
+                continue
+
+            matched_count += 1
+            if line not in seen:
+                seen.add(line)
+                result.append(line)
+
+    destination.parent.mkdir(parents=True, exist_ok=True)
+    with destination.open("w", encoding="utf-8", newline="\n") as output_file:
+        output_file.write("\n".join(result))
+        if result:
+            output_file.write("\n")
+
+    return matched_count, len(result)
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(
+        description="筛选辅助码恰好为两位字符的记录，并按完整记录去重。"
+    )
+    parser.add_argument("input", nargs="?", type=Path, default=DEFAULT_INPUT)
+    parser.add_argument("output", nargs="?", type=Path, default=DEFAULT_OUTPUT)
+    args = parser.parse_args()
+
+    matched, written = filter_helpcode(args.input, args.output)
+    print(f"读取文件：{args.input}")
+    print(f"两位辅助码记录：{matched} 条")
+    print(f"去重后写入：{written} 条")
+    print(f"输出文件：{args.output}")
+
+
+if __name__ == "__main__":
+    main()

engine/helpcode/scripts/filter_zrm_helpcode_small.py

@@ -0,0 +1,69 @@
+"""根据 helpcode.txt 中已有的汉字筛选 small 版自然码辅助码表。"""
+
+from __future__ import annotations
+
+import argparse
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parent.parent
+DEFAULT_INPUT = ROOT / "helpcodes" / "zrm_helpcode_big.txt"
+DEFAULT_REFERENCE = ROOT / "helpcodes" / "helpcode.txt"
+DEFAULT_OUTPUT = ROOT / "helpcodes" / "zrm_helpcode.txt"
+
+
+def read_keys(path: Path) -> set[str]:
+    """读取等号左侧的全部键（汉字或词条）。"""
+    keys: set[str] = set()
+    with path.open("r", encoding="utf-8-sig") as file:
+        for raw_line in file:
+            line = raw_line.rstrip("\r\n")
+            if "=" in line:
+                key, _ = line.split("=", 1)
+                keys.add(key)
+    return keys
+
+
+def create_small_table(source: Path, reference: Path, output: Path) -> tuple[int, int]:
+    """保留 source 中键存在于 reference 的所有记录，按完整行去重。"""
+    wanted_keys = read_keys(reference)
+    seen: set[str] = set()
+    result: list[str] = []
+
+    with source.open("r", encoding="utf-8-sig") as file:
+        for raw_line in file:
+            line = raw_line.rstrip("\r\n")
+            if "=" not in line:
+                continue
+            key, _ = line.split("=", 1)
+            if key in wanted_keys and line not in seen:
+                seen.add(line)
+                result.append(line)
+
+    with output.open("w", encoding="utf-8", newline="\n") as file:
+        file.write("\n".join(result))
+        if result:
+            file.write("\n")
+
+    return len(wanted_keys), len(result)
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(
+        description="按 helpcode.txt 已有汉字筛选 small 版辅助码表。"
+    )
+    parser.add_argument("input", nargs="?", type=Path, default=DEFAULT_INPUT)
+    parser.add_argument("reference", nargs="?", type=Path, default=DEFAULT_REFERENCE)
+    parser.add_argument("output", nargs="?", type=Path, default=DEFAULT_OUTPUT)
+    args = parser.parse_args()
+
+    key_count, line_count = create_small_table(
+        args.input, args.reference, args.output
+    )
+    print(f"参考汉字：{key_count} 个")
+    print(f"写入记录：{line_count} 条")
+    print(f"输出文件：{args.output}")
+
+
+if __name__ == "__main__":
+    main()

engine/helpcode/scripts/lcopyHelpcode.ps1

@@ -0,0 +1,3 @@
+$target = Join-Path $env:LOCALAPPDATA 'metasequoiaime\helpcodes'
+New-Item -ItemType Directory -Path $target -Force | Out-Null
+Copy-Item -Path ".\helpcodes\*" -Destination $target -Recurse -Force