fix: 跨文件KEY约束 + PERFORM分支统计 + 平面文件写入

1. 跨文件KEY约束(修复)
   匹配型程的M-KEY与D-KEY值不同导致匹配0条。
   修复: generate_data后处理检测IF KEY比较,
   前半记录对齐KEY值(8条匹配),后半保待差异(9条不匹配).
   实际cobc运行验证: MATCHED=8, PASS.

2. extract_structure PERFORM分支统计(修复)
   _walk函数未添加BrPerform决策点, total_branches缺失.
   修复: 为PERFORM UNTIL/VARYING决策点添加2分支(Enter/Skip).
   之前total_branches=0,现在=2.

3. flatfile.py(新增)
   COBOL固定长平面文件写入器.
   - analyze_fd_layout(): 从COBOL源码自动解析文件布局
   - write_flat_file(): 生成为COBOL可直接读取的二进制格式

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

cobol_testgen/flatfile.py

@@ -0,0 +1,145 @@
+"""Flat file I/O — write fixed-length records from COBOL FD definitions"""
+import struct, re
+from pathlib import Path
+from typing import Any
+
+def analyze_fd_layout(source_text: str) -> dict[str, dict]:
+    """From preprocessed COBOL source, extract FD file layouts.
+
+    Returns: {assign_to_filename: {
+        "fd_name": str,
+        "records": [{record_name, record_length, fields: [{name, offset, length, type, pic}]}],
+        "direction": "INPUT"|"OUTPUT"|"I-O"
+    }}
+    """
+    from .read import parse_file_control, parse_file_section, parse_data_division, extract_data_division, scan_open_statements
+
+    fc = parse_file_control(source_text) if source_text else {}
+    fs = parse_file_section(source_text) if source_text else {}
+    ops = scan_open_statements(source_text) if source_text else {}
+    dd = extract_data_division(source_text)
+    all_fields = parse_data_division(dd) if dd else []
+
+    layouts = {}
+    for fd_name, rec_names in fs.items():
+        records = []
+        for rec_name in rec_names:
+            children = []
+            found = False
+            rec_level = None
+            offset = 0
+            for f in all_fields:
+                if f.name == rec_name:
+                    found = True
+                    rec_level = f.level
+                    continue
+                if found:
+                    if f.level is not None and f.level <= rec_level:
+                        break
+                    if f.is_88 or f.is_filler:
+                        continue
+                    pi = f.pic_info
+                    if pi:
+                        length = (pi.digits + pi.decimal) if pi.type == "numeric" else (pi.length or 0)
+                    else:
+                        length = 0
+                    ftype = pi.type if pi else "unknown"
+                    children.append({
+                        "name": f.name, "pic": str(f.pic or ""),
+                        "type": ftype, "length": length, "offset": offset,
+                    })
+                    offset += length
+
+            records.append({"record_name": rec_name, "fields": children, "record_length": offset})
+
+        assign_to = fc.get(fd_name, {}).get("assign_to", fd_name)
+        layouts[assign_to] = {
+            "fd_name": fd_name, "records": records,
+            "direction": ops.get(fd_name, "INPUT"),
+        }
+    return layouts
+
+
+def _format_value(value: Any, field: dict) -> bytes:
+    """Format a value for COBOL fixed-length storage."""
+    ftype = field["type"]
+    length = field["length"]
+    val = str(value) if value is not None else ""
+
+    if ftype == "numeric":
+        # COBOL numeric DISPLAY format: right-justified, zero-padded
+        try:
+            num = int(float(val)) if val else 0
+        except (ValueError, TypeError):
+            num = 0
+        num = abs(num)
+        s = str(num).zfill(length)
+        if len(s) > length:
+            s = s[-length:]
+        return s.encode("ascii")
+    else:
+        # Alphanumeric: left-justified, space-padded
+        s = val.ljust(length)[:length]
+        return s.encode("ascii", errors="replace")
+
+
+def _format_comp3(value: Any, field: dict) -> bytes:
+    """Format as COMP-3 (packed decimal)."""
+    length = field["length"]
+    val = str(value) if value else "0"
+    try:
+        num_str = val.replace(".", "").lstrip("0") or "0"
+        if num_str.startswith("-"):
+            sign_nibble = 0x0D
+            num_str = num_str[1:]
+        else:
+            sign_nibble = 0x0C
+        if len(num_str) % 2 == 1:
+            num_str = "0" + num_str
+        result = []
+        for i in range(0, len(num_str), 2):
+            result.append(int(num_str[i:i+2], 16))
+        result[-1] = (result[-1] & 0xF0) | sign_nibble
+        return bytes(result)
+    except (ValueError, TypeError):
+        return bytes([0x00] * (length // 2 + 1 if length % 2 else length // 2))
+
+
+def write_flat_file(records: list[dict], layout: dict, outpath: Path):
+    """Write generated records as a COBOL-compatible fixed-length flat file.
+
+    Args:
+        records: list of dicts with field_name: value
+        layout: file layout from analyze_fd_layout()
+        outpath: output file path
+    """
+    if not layout or not layout.get("records"):
+        return
+
+    rec = layout["records"][0]  # Use first record format
+    rec_len = rec["record_length"]
+    if rec_len == 0:
+        return
+
+    with open(outpath, "wb") as f:
+        for row in records:
+            buf = bytearray(rec_len)
+            for field in rec["fields"]:
+                val = row.get(field["name"], "")
+                formatted = _format_value(val, field)
+                end = min(field["offset"] + len(formatted), rec_len)
+                buf[field["offset"]:end] = formatted[:end - field["offset"]]
+            f.write(buf)
+
+
+def write_all_files(records: list[dict], source_text: str, outdir: Path, prefix: str = ""):
+    """Analyze source, write flat files for all FDs that have data."""
+    layouts = analyze_fd_layout(source_text)
+    written = []
+    for filename, layout in layouts.items():
+        if layout["direction"] == "OUTPUT":
+            continue  # Don't write output files (COBOL will create them)
+        outpath = outdir / (prefix + filename)
+        write_flat_file(records, layout, outpath)
+        written.append((filename, outpath))
+    return written