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/conftest.py

@@ -0,0 +1,49 @@
+"""Shared test helpers: a mock ESI client backed by httpx.MockTransport."""
+
+from __future__ import annotations
+
+from collections.abc import Callable
+
+import httpx
+import pytest
+
+from familiar import esi
+
+Handler = Callable[[httpx.Request], httpx.Response]
+
+
+@pytest.fixture
+def mock_esi():
+    """Install a mock ESI client driven by a request handler.
+
+    Usage::
+
+        def test_x(mock_esi):
+            def handler(request):
+                return httpx.Response(200, json={...})
+            mock_esi(handler)
+            ...
+
+    The shared client is reset after the test.
+    """
+
+    def _make(handler: Handler) -> esi.EsiClient:
+        client = esi.EsiClient(transport=httpx.MockTransport(handler))
+        esi.set_client(client)
+        return client
+
+    yield _make
+    esi.set_client(None)
+
+
+def route(request: httpx.Request, fixtures: dict[str, object]) -> httpx.Response:
+    """Match a request against ``{substring_in_path: json_body}`` and respond.
+
+    The special key ``"/universe/ids/"`` is matched for the name-resolution POST.
+    A missing match returns 404 so tests fail loudly.
+    """
+    path = request.url.path
+    for needle, body in fixtures.items():
+        if needle in path:
+            return httpx.Response(200, json=body)
+    return httpx.Response(404, json={"error": f"no fixture for {path}"})