fix: band-level windowed refine_layout + programmatic map_fields to prevent 91.5% content loss

Root cause: LLM receiving full 34k-char JRXML would regenerate from scratch
instead of modifying coordinates in-place, shrinking output to ~3k chars.

Solution (programmatic node control, not prompt engineering):

- New agent/jrxml_windower.py: decompose JRXML into header (never sent to
  LLM) + individual bands. Split bands >4000 chars at element boundaries.
  Reassemble with element count validation (>10% change = rollback).

- Rewrite refine_layout: per-band windowed LLM processing (~2-4k chars
  each). LLM cannot "reimagine" the entire report.

- Rewrite map_fields: 100% programmatic regex $F{field_N} -> real name
  replacement. Zero LLM calls, zero content loss.

- _sanitize_field_name: non-ASCII chars escaped to _uXXXX_ format for
  valid JRXML identifiers.

- Tests: 48 new unit tests (windower 28 + map_fields 20). All passing.
  Full suite 385 tests, zero regressions.

scripts/init_default_kb.py

@@ -0,0 +1,134 @@
+"""初始化默认用户和预置知识库。
+
+解析 rag/jrxml_source/ 下的全部 JRXML 模板 + 接口文档，
+创建默认用户 "默认用户" 和知识库 "F6-汽车维修打印默认知识库"，
+执行完整的 parse -> chunk -> embed 管线。
+
+用法:
+    python scripts/init_default_kb.py
+    python scripts/init_default_kb.py --force  # 强制重建
+"""
+
+import os
+import sys
+import argparse
+from pathlib import Path
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(PROJECT_ROOT))
+
+from dotenv import load_dotenv
+load_dotenv()
+
+from backend.logger import get_logger
+
+_log = get_logger("init_kb")
+
+DEFAULT_USER_NAME = "默认用户"
+DEFAULT_KB_NAME = "F6-汽车维修打印默认知识库"
+DEFAULT_KB_DESC = "预置的汽车维修打印单模板与接口文档知识库"
+
+
+def find_source_files(source_dir: Path) -> list[str]:
+    """收集源目录下的所有 .jrxml 和 .md 文件。"""
+    files = []
+    if not source_dir.exists():
+        _log.warning("源目录不存在: %s", source_dir)
+        return files
+
+    for pattern in ("*.jrxml", "*.md", "**/*.jrxml", "**/*.md"):
+        for fp in source_dir.glob(pattern):
+            if fp.is_file():
+                files.append(str(fp))
+
+    return list(dict.fromkeys(files))
+
+
+def init_default_kb(force: bool = False) -> dict:
+    """初始化默认 KB，返回结果摘要。"""
+    from backend.kb_manager import create_user, list_users, create_kb, list_kbs
+    from backend.kb_parser import build_kb_from_files
+
+    # 1. 查找或创建默认用户
+    users = list_users()
+    default_user = None
+    for u in users:
+        if u.get("name") == DEFAULT_USER_NAME:
+            default_user = u
+            break
+
+    if default_user is None:
+        default_user = create_user(DEFAULT_USER_NAME)
+        _log.info("创建默认用户: %s", default_user["user_id"])
+    else:
+        _log.info("默认用户已存在: %s", default_user["user_id"])
+
+    user_id = default_user["user_id"]
+
+    # 2. 查找或创建默认 KB
+    user_kbs = list_kbs(user_id)
+    default_kb = None
+    for kb in user_kbs:
+        if kb.get("name") == DEFAULT_KB_NAME:
+            default_kb = kb
+            break
+
+    if default_kb is not None and force:
+        from backend.kb_manager import delete_kb
+        delete_kb(default_kb["kb_id"])
+        default_kb = None
+        _log.info("强制重建: 已删除旧 KB")
+
+    if default_kb is None:
+        default_kb = create_kb(user_id, DEFAULT_KB_NAME, DEFAULT_KB_DESC)
+        _log.info("创建默认知识库: %s", default_kb["kb_id"])
+    elif default_kb.get("parse_status") == "ready":
+        _log.info("默认知识库已就绪: chunks=%s", default_kb.get("chunk_count", 0))
+        return {"status": "already_ready", "kb_id": default_kb["kb_id"],
+                "user_id": user_id, "chunk_count": default_kb.get("chunk_count", 0)}
+
+    kb_id = default_kb["kb_id"]
+
+    # 3. 收集源文件
+    source_dir = PROJECT_ROOT / "rag" / "jrxml_source"
+    files = find_source_files(source_dir)
+    if not files:
+        _log.warning("未找到源文件，跳过构建")
+        return {"status": "no_files", "kb_id": kb_id, "user_id": user_id}
+
+    _log.info("找到 %d 个源文件", len(files))
+
+    # 4. 构建 KB
+    result = build_kb_from_files(kb_id, files)
+    _log.info("KB 构建完成: %s", result)
+
+    return {**result, "kb_id": kb_id, "user_id": user_id, "file_count": len(files)}
+
+
+def main():
+    parser = argparse.ArgumentParser(description="初始化默认知识库")
+    parser.add_argument("--force", action="store_true", help="强制重建（删除已有 KB）")
+    args = parser.parse_args()
+
+    print("=" * 60)
+    print("JRXML Agent - 默认知识库初始化")
+    print("=" * 60)
+
+    result = init_default_kb(force=args.force)
+
+    print(f"\n用户: {DEFAULT_USER_NAME}")
+    print(f"知识库: {DEFAULT_KB_NAME}")
+    print(f"状态: {result.get('status', 'unknown')}")
+    print(f"字段数: {result.get('field_count', 0)}")
+    print(f"模板数: {result.get('template_count', 0)}")
+    print(f"Chunk数: {result.get('chunk_count', 0)}")
+    if result.get("errors"):
+        print(f"错误: {len(result['errors'])} 条")
+        for e in result["errors"]:
+            print(f"  - {e.get('file', '')}: {e.get('error', '')}")
+
+    print("\n初始化完成!")
+
+
+if __name__ == "__main__":
+    main()