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.

tests/test_readonly_chart_endpoints.py

@@ -0,0 +1,158 @@
+"""HTTP contracts for POST /api/western and POST /api/ephemeris_events."""
+
+from __future__ import annotations
+
+import json
+from io import BytesIO
+
+import pytest
+
+from scripts.api_heavy_compute_gate import reset_heavy_compute_gate
+from scripts.jyotish_api_server import (
+    DEFAULT_ALLOWED_HOSTS,
+    DEFAULT_ALLOWED_ORIGINS,
+    JyotishAPIHandler,
+)
+
+SAMPLE = {
+    "year": 1990,
+    "month": 4,
+    "day": 9,
+    "hour": 13,
+    "minute": 24,
+    "lat": 31.19,
+    "lon": 121.44,
+    "tz": 8,
+}
+
+
+class _FakeHeaders(dict):
+    def get(self, key, default=None):
+        return super().get(key, default)
+
+
+class _FakeServer:
+    allowed_origins = DEFAULT_ALLOWED_ORIGINS
+    allowed_hosts = DEFAULT_ALLOWED_HOSTS
+
+
+class _PostCaptureHandler(JyotishAPIHandler):
+    def __init__(self, path: str, payload: dict) -> None:
+        raw = json.dumps(payload).encode("utf-8")
+        self.headers = _FakeHeaders(
+            {
+                "Content-Length": str(len(raw)),
+                "Content-Type": "application/json",
+            }
+        )
+        self.server = _FakeServer()
+        self.path = path
+        self.rfile = BytesIO(raw)
+        self.wfile = BytesIO()
+        self.status_code = None
+        self.response_headers = []
+        self.client_address = ("test-readonly-chart", 0)
+
+    def send_response(self, code, message=None):  # noqa: ANN001
+        self.status_code = code
+
+    def send_header(self, key, value):  # noqa: ANN001
+        self.response_headers.append((key, value))
+
+    def end_headers(self):
+        return None
+
+    def payload(self) -> dict:
+        return json.loads(self.wfile.getvalue().decode("utf-8"))
+
+
+@pytest.fixture
+def api_env(monkeypatch):
+    monkeypatch.setenv("JYOTISH_API_RATE_LIMIT_PER_MINUTE", "0")
+    reset_heavy_compute_gate()
+    yield
+    reset_heavy_compute_gate()
+
+
+def test_western_returns_planets_cusps_aspects_and_distribution(api_env) -> None:
+    handler = _PostCaptureHandler("/api/western", SAMPLE)
+    handler.do_POST()
+    assert handler.status_code == 200
+    payload = handler.payload()
+    natal = payload["natal"]
+    assert payload["zodiac"] == "tropical"
+    assert payload["house_system"] == "P"
+    assert payload["coordinate_system"] == "tropical_ecliptic"
+    assert len(natal["planets"]) == 11
+    assert len(natal["houses"]) == 12
+    assert natal["aspects"]
+    assert set(natal["distribution"]["elements"]) == {"fire", "earth", "air", "water"}
+    assert "岁差" in payload["boundary"] or "ayanamsa" in json.dumps(payload["sidereal_offset_degrees"])
+
+
+def test_western_house_system_can_be_overridden_and_echoed(api_env) -> None:
+    handler = _PostCaptureHandler("/api/western", {**SAMPLE, "house_system": "K"})
+    handler.do_POST()
+    assert handler.status_code == 200
+    payload = handler.payload()
+    assert payload["house_system"] == "K"
+    placidus = _PostCaptureHandler("/api/western", SAMPLE)
+    placidus.do_POST()
+    assert placidus.payload()["house_system"] == "P"
+    assert (
+        placidus.payload()["natal"]["houses"][2]["cusp"]["longitude"]
+        != payload["natal"]["houses"][2]["cusp"]["longitude"]
+    )
+
+
+def test_ephemeris_events_90_day_window_has_sorted_ingress_and_station(api_env) -> None:
+    handler = _PostCaptureHandler(
+        "/api/ephemeris_events",
+        {
+            "start_date": "2026-09-15",
+            "end_date": "2026-12-14",
+            "ayanamsa": "raman",
+            "node_mode": "mean",
+        },
+    )
+    handler.do_POST()
+    assert handler.status_code == 200
+    payload = handler.payload()
+    events = payload["events"]
+    kinds = {item["kind"] for item in events}
+    assert "ingress" in kinds
+    assert "station" in kinds
+    dates = [item["date"] for item in events]
+    assert dates == sorted(dates)
+    assert payload["calculation"]["ayanamsa"] == "raman"
+    assert len(events) == 14
+
+
+def test_ephemeris_events_rejects_range_over_730_days(api_env) -> None:
+    handler = _PostCaptureHandler(
+        "/api/ephemeris_events",
+        {"start_date": "2026-01-01", "end_date": "2028-01-02", "ayanamsa": "raman"},
+    )
+    handler.do_POST()
+    assert handler.status_code == 400
+    payload = handler.payload()
+    assert payload["success"] is False
+    assert "730" in payload["error"]
+
+
+def test_ephemeris_ayanamsa_changes_the_event_list(api_env) -> None:
+    raman = _PostCaptureHandler(
+        "/api/ephemeris_events",
+        {"start_date": "2026-09-15", "end_date": "2026-12-14", "ayanamsa": "raman"},
+    )
+    lahiri = _PostCaptureHandler(
+        "/api/ephemeris_events",
+        {"start_date": "2026-09-15", "end_date": "2026-12-14", "ayanamsa": "lahiri"},
+    )
+    raman.do_POST()
+    lahiri.do_POST()
+    assert raman.status_code == 200
+    assert lahiri.status_code == 200
+    assert raman.payload()["events"] != lahiri.payload()["events"]
+    assert raman.payload()["calculation"]["ayanamsa"] == "raman"
+    assert lahiri.payload()["calculation"]["ayanamsa"] == "lahiri"