Phase 1: public market + universe lookups via ESI

Add a thin async ESI client (httpx, descriptive User-Agent, uniform errors,
mockable transport) plus name<->id resolution. New read-only tools:

- universe: resolve_names, get_item, get_system, get_station, get_region, get_route
- market:   list_hubs, get_market_price, get_market_orders, compare_hubs

Trade-hub IDs (Jita/Amarr/Dodixie/Rens/Hek) are hardcoded constants. Market
aggregation (best buy/sell, spread, order book) lives in pure helpers. 30 unit
tests with mocked ESI; coverage 99%. SDE bulk-load deferred to a later phase.

Closes #2

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_esi.py

@@ -0,0 +1,56 @@
+"""ESI client: error handling, empty bodies, datasource param."""
+
+import asyncio
+
+import httpx
+import pytest
+
+from familiar import esi
+
+
+def test_get_raises_esi_error_on_http_error(mock_esi):
+    mock_esi(lambda req: httpx.Response(404, json={"error": "not found"}))
+    with pytest.raises(esi.EsiError) as exc:
+        asyncio.run(esi.client().get("/universe/types/999999/"))
+    assert exc.value.status == 404
+    assert "404" in str(exc.value)
+
+
+def test_get_raises_on_network_error(mock_esi):
+    def boom(req):
+        raise httpx.ConnectError("boom")
+
+    mock_esi(boom)
+    with pytest.raises(esi.EsiError):
+        asyncio.run(esi.client().get("/anything/"))
+
+
+def test_empty_body_returns_none(mock_esi):
+    mock_esi(lambda req: httpx.Response(200, content=b""))
+    assert asyncio.run(esi.client().get("/x/")) is None
+
+
+def test_datasource_param_is_attached(mock_esi):
+    seen = {}
+
+    def handler(req):
+        seen["datasource"] = req.url.params.get("datasource")
+        return httpx.Response(200, json={"ok": True})
+
+    mock_esi(handler)
+    asyncio.run(esi.client().get("/x/"))
+    assert seen["datasource"] == "tranquility"
+
+
+def test_error_detail_falls_back_to_text(mock_esi):
+    mock_esi(lambda req: httpx.Response(500, content=b"upstream exploded"))
+    with pytest.raises(esi.EsiError) as exc:
+        asyncio.run(esi.client().get("/x/"))
+    assert "upstream exploded" in str(exc.value)
+
+
+def test_default_client_is_singleton():
+    esi.set_client(None)
+    first = esi.client()
+    assert esi.client() is first
+    esi.set_client(None)