feat(qizheng): vendor stem-branch and expose three read-only chart APIs

Add Apache-2.0 @4n6h4x0r/stem-branch 0.8.0, a seven-governors adapter,
and POST /api/qizheng, /api/western, /api/ephemeris_events.

BUG-700 remains blocked (do not call --pillars). BUG-701 and BUG-702 are
resolved. BUG-703 is investigating (panchanga Lahiri). Skill is not bumped.

scripts/western_chart_engine.py

@@ -21,6 +21,10 @@ try:
     from western_evidence_packet import build_western_evidence_packet
 except ImportError:  # pragma: no cover - package import path
     from scripts.western_evidence_packet import build_western_evidence_packet
+try:
+    from ayanamsa_utils import DEFAULT_AYANAMSA_NAME, ayanamsa_display_name, temporary_ayanamsa
+except ImportError:  # pragma: no cover - package import path
+    from scripts.ayanamsa_utils import DEFAULT_AYANAMSA_NAME, ayanamsa_display_name, temporary_ayanamsa
 
 
 _PLANETS = {
@@ -213,6 +217,50 @@ def build_tropical_natal_chart(
     }
 
 
+def _request_field(body: dict[str, Any], *keys: str) -> Any:
+    for key in keys:
+        if key in body and body[key] not in (None, ""):
+            return body[key]
+    raise ValueError(f"{keys[0]} is required")
+
+
+def build_tropical_natal_from_request(body: dict[str, Any]) -> dict[str, Any]:
+    """Read-only tropical natal chart for POST /api/western."""
+    if not isinstance(body, dict):
+        raise ValueError("JSON body must be an object")
+    house_system = str(body.get("house_system") or body.get("houseSystem") or "P").strip() or "P"
+    chart = build_tropical_natal_chart(
+        year=int(_request_field(body, "year")),
+        month=int(_request_field(body, "month")),
+        day=int(_request_field(body, "day")),
+        hour=int(_request_field(body, "hour")),
+        minute=int(_request_field(body, "minute")),
+        second=int(body.get("second") or 0),
+        latitude=float(_request_field(body, "lat", "latitude")),
+        longitude=float(_request_field(body, "lon", "longitude", "lng")),
+        timezone=_request_field(body, "tz", "timezone"),
+        house_system=house_system,
+    )
+    jd_ut = float(chart["calculation_contract"]["julian_day_ut"])
+    with temporary_ayanamsa(DEFAULT_AYANAMSA_NAME):
+        ayanamsa_degrees = round(float(swe.get_ayanamsa_ut(jd_ut)), 6)
+    chart["coordinate_system"] = "tropical_ecliptic"
+    chart["sidereal_offset_degrees"] = {
+        "reference": DEFAULT_AYANAMSA_NAME,
+        "display": ayanamsa_display_name(DEFAULT_AYANAMSA_NAME),
+        "degrees": ayanamsa_degrees,
+    }
+    chart["boundary"] = (
+        "这是回归黄道（分点黄经），不是恒星黄道，也不是七政宿度。"
+        f"{ayanamsa_display_name(DEFAULT_AYANAMSA_NAME)} 岁差在此刻约 {ayanamsa_degrees:.4f}°。"
+        "三套坐标系不得互相换算，也不得叠加成双重印证。"
+        " Natal tropical calculation only; it does not calculate transits, progressions, solar arcs, returns, or interpretation."
+    )
+    chart["success"] = True
+    chart["endpoint"] = "western"
+    return chart
+
+
 def build_tropical_western_evidence_packet(*, route_packet: dict[str, Any], **birth: Any) -> dict[str, Any]:
     """Wrap direct natal calculation in the existing cross-system packet contract."""
     chart = build_tropical_natal_chart(**birth)