tools/epro2/kicad: Phase-1 EPRO2 → KiCad schematic exporter

写第一版 EPRO2 → .kicad_sch 转换：把 SCH_PAGE Document 的 wires +
COMPONENT placements + TEXT 输出到一个可被 KiCad 7+ 打开的 sch 文件。
不含 symbol 主体（lib_symbols 留空 stub），所以 KiCad 里组件会渲染
成红色 "?" 占位，但布线 + 位置 + Designator/Value 属性都正确。完整
symbol 库导出留 Phase 2。

模块结构：
  tools/epro2/kicad/sexpr.py        手写 S-expr emitter，Sym 标记裸符号，
                                    str 自动加引号 + 转义；float 去尾零；
                                    bool→yes/no；NaN/Inf 主动报错
  tools/epro2/kicad/_sexpr_reader.py  极简 S-expr parser，仅给 round-trip
                                    测试用（非完整 KiCad reader）
  tools/epro2/kicad/sch_writer.py   write_sch_page(doc) → str；处理：
                                      LINE  → (wire (pts ...) ...)
                                      COMPONENT → (symbol (lib_id facere:<partId>)
                                                  (at x y rot) (property Reference ...) ...)
                                      TEXT  → (text "..." (at ...))
                                    单位 mil → mm × 0.0254；零长 wire 跳过
  tools/epro2/kicad/__main__.py     CLI: --doc <uuid> | --all-sch

ESP-VoCat 验证（python -m tools.epro2.kicad <project> --all-sch）：
  9 SCH_PAGE 全部转换成功
  P1_408c9f4f.kicad_sch    wires=  6  symbols= 10  text=  0  skipped= 2  (370 lines)
  P1_ee409917.kicad_sch    wires= 20  symbols= 14  text=  0  skipped= 3
  P1_54743d77.kicad_sch    wires= 42  symbols= 30  text=  3
  Overview_dc13d6d2.kicad_sch wires=  0  symbols=  1  text= 34   (说明页)
  MCU_510cff33.kicad_sch   wires= 91  symbols= 86  text=  9
  Interface_b336a7c7.kicad_sch wires= 99  symbols= 95  text=  6
  P1_5c38f45b.kicad_sch    wires=179  symbols= 86  text=  9
  P1_45092758.kicad_sch    wires=187  symbols=138  text= 10  (主图)
  codec_0b0163fa.kicad_sch wires=190  symbols=112  text= 10

输出落在 data/processed/kicad_sch/<filename>.kicad_sch（gitignore 内，
可重新生成；不入库）。

测试：6 个 sexpr 测 + 6 个 sch_writer 测，含 round-trip parse 验证。
parser/relations/project_relations 的旧 21 个不动，合计 **33/33 通过**。

下一步：
1. Phase 2 — symbol library 导出 (.kicad_sym)，把 SYMBOL doc 的 PIN/RECT/
   TEXT primitives 转 KiCad symbol 主体；填 lib_symbols 块让组件渲染
   出真正的 schematic 符号
2. footprint library + .kicad_pcb 导出
3. 用 KiCad CLI (kicad-cli sch erc) 跑 ERC 校验

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/epro2/kicad/sexpr.py

@@ -0,0 +1,75 @@
+"""Hand-rolled S-expression emitter for KiCad files.
+
+KiCad reads a Lisp-flavored S-expr; values are atoms (symbols / strings /
+numbers) and lists. We emit lists as nested Python lists, with ``Sym`` marking
+strings that should serialize as bare symbols (unquoted) — e.g. ``Sym("xy")``,
+``Sym("kicad_sch")``, ``Sym("yes")``.
+
+Plain ``str`` values get JSON-style double-quoting + backslash escaping.
+Floats render with up to 6 decimals trimmed of trailing zeros (KiCad reads
+either form, but trimmed is what kicad-cli emits).
+"""
+
+from __future__ import annotations
+
+import io
+import math
+from typing import Any
+
+
+class Sym(str):
+    """Marker subclass: render as a bare S-expr symbol (unquoted)."""
+
+    __slots__ = ()
+
+
+def _fmt_number(n: float | int) -> str:
+    if isinstance(n, bool):  # bool is int subclass — guard first
+        return "yes" if n else "no"
+    if isinstance(n, int):
+        return str(n)
+    if math.isnan(n) or math.isinf(n):
+        raise ValueError(f"can't serialize non-finite number: {n}")
+    s = f"{n:.6f}".rstrip("0").rstrip(".")
+    return s if s else "0"
+
+
+def _emit(value: Any, out: io.StringIO, indent: int, *, pretty: bool) -> None:
+    if isinstance(value, list):
+        out.write("(")
+        for i, item in enumerate(value):
+            if i > 0:
+                if pretty and isinstance(item, list) and len(item) > 1:
+                    out.write("\n" + "\t" * (indent + 1))
+                else:
+                    out.write(" ")
+            _emit(item, out, indent + 1, pretty=pretty)
+        out.write(")")
+    elif isinstance(value, Sym):
+        out.write(str(value))
+    elif value is True or value is False:
+        out.write("yes" if value else "no")
+    elif isinstance(value, (int, float)):
+        out.write(_fmt_number(value))
+    elif isinstance(value, str):
+        escaped = value.replace("\\", "\\\\").replace('"', '\\"')
+        out.write(f'"{escaped}"')
+    elif value is None:
+        # rare but sometimes a slot is intentionally empty
+        out.write('""')
+    else:
+        raise TypeError(f"can't S-expr serialize {type(value).__name__}: {value!r}")
+
+
+def to_sexpr(value: Any, *, pretty: bool = True) -> str:
+    """Render a Python data structure as a KiCad-style S-expression.
+
+    Top-level value is usually a list whose first element is a ``Sym`` (the
+    block tag, e.g. ``Sym("kicad_sch")``). When ``pretty=True``, child lists
+    of length > 1 go onto their own indented line.
+    """
+    buf = io.StringIO()
+    _emit(value, buf, indent=0, pretty=pretty)
+    if pretty:
+        buf.write("\n")
+    return buf.getvalue()