tests: add minimal coverage for percentile + proxy routing (S1)

- tests/test_metrics.py asserts the new linear-interp _percentile against
  hand-computed expected values (single value, two-value interpolation,
  endpoints, numpy-equivalent linear default, on-integer rank).
- tests/test_proxy_pick.py exercises InstanceState LRU eviction and
  move-to-end on hit, plus session-affinity stickiness, the overload
  fallback, the active_p_offloads penalty, and lmetric scoring. The
  proxy is loaded by file path with stub fastapi/uvicorn/httpx modules
  so the suite runs without the FastAPI server deps installed.
- pyproject.toml gets a hatchling wheel target and a [tool.pytest]
  section so `uv run --extra dev pytest` works out of the box.

tests/test_proxy_pick.py

@@ -0,0 +1,180 @@
+"""Minimal coverage for scripts/cache_aware_proxy pick_instance + cache LRU (S1)."""
+
+from __future__ import annotations
+
+import importlib.util
+import sys
+import types
+from pathlib import Path
+
+import pytest
+
+PROXY_PATH = Path(__file__).resolve().parent.parent / "scripts" / "cache_aware_proxy.py"
+
+
+def _install_stub_modules() -> None:
+    """Provide minimal stand-ins for fastapi/uvicorn/httpx so the proxy
+    module imports cleanly without the full server deps."""
+    if "uvicorn" not in sys.modules:
+        sys.modules["uvicorn"] = types.ModuleType("uvicorn")
+
+    if "fastapi" not in sys.modules:
+        fastapi_mod = types.ModuleType("fastapi")
+
+        class _FastAPI:
+            def __init__(self, *a, **kw):
+                self.state = types.SimpleNamespace()
+
+            def post(self, *a, **kw):
+                def deco(fn): return fn
+                return deco
+
+            def get(self, *a, **kw):
+                def deco(fn): return fn
+                return deco
+
+        class _HTTPException(Exception):
+            def __init__(self, status_code=500, detail=""):
+                self.status_code = status_code
+                self.detail = detail
+
+        class _Request:  # not actually instantiated by the routing tests
+            pass
+
+        fastapi_mod.FastAPI = _FastAPI
+        fastapi_mod.HTTPException = _HTTPException
+        fastapi_mod.Request = _Request
+        sys.modules["fastapi"] = fastapi_mod
+
+        responses_mod = types.ModuleType("fastapi.responses")
+
+        class _StreamingResponse:
+            def __init__(self, *a, **kw): pass
+
+        responses_mod.StreamingResponse = _StreamingResponse
+        sys.modules["fastapi.responses"] = responses_mod
+
+    if "httpx" not in sys.modules:
+        httpx_mod = types.ModuleType("httpx")
+
+        class _AsyncClient:
+            def __init__(self, *a, **kw): pass
+            async def aclose(self): pass
+
+        class _Limits:
+            def __init__(self, *a, **kw): pass
+
+        httpx_mod.AsyncClient = _AsyncClient
+        httpx_mod.Limits = _Limits
+        sys.modules["httpx"] = httpx_mod
+
+
+@pytest.fixture(scope="module")
+def proxy():
+    _install_stub_modules()
+    spec = importlib.util.spec_from_file_location("cache_aware_proxy", PROXY_PATH)
+    if spec is None or spec.loader is None:
+        pytest.skip(f"cannot load proxy module at {PROXY_PATH}")
+    mod = importlib.util.module_from_spec(spec)
+    sys.modules["cache_aware_proxy"] = mod
+    try:
+        spec.loader.exec_module(mod)
+    except ModuleNotFoundError as exc:
+        pytest.skip(f"proxy dependency missing: {exc}")
+    return mod
+
+
+def _make_inst(proxy, url: str, ongoing_tokens: int = 0,
+               active_p_offloads: int = 0):
+    inst = proxy.InstanceState(url)
+    inst.ongoing_tokens = ongoing_tokens
+    inst.active_p_offloads = active_p_offloads
+    return inst
+
+
+def test_record_prefix_evicts_oldest_block(proxy):
+    """LRU bound on cached_blocks must evict the oldest entry once full."""
+    inst = proxy.InstanceState("http://x")
+    saved = proxy.CACHE_CAPACITY_BLOCKS
+    proxy.CACHE_CAPACITY_BLOCKS = 2
+    try:
+        block_size = proxy.BLOCK_SIZE
+        # Three distinct one-block prefixes; first must be evicted.
+        prefix_a = [1] * block_size
+        prefix_b = [2] * block_size
+        prefix_c = [3] * block_size
+        inst.record_prefix(prefix_a)
+        inst.record_prefix(prefix_b)
+        inst.record_prefix(prefix_c)
+        assert len(inst.cached_blocks) == 2
+        # A should have been evicted.
+        assert inst.estimate_cache_hit(prefix_a) == 0
+        assert inst.estimate_cache_hit(prefix_b) == block_size
+        assert inst.estimate_cache_hit(prefix_c) == block_size
+    finally:
+        proxy.CACHE_CAPACITY_BLOCKS = saved
+
+
+def test_estimate_cache_hit_touches_lru(proxy):
+    """A cache hit must move the block to the MRU position."""
+    inst = proxy.InstanceState("http://x")
+    saved = proxy.CACHE_CAPACITY_BLOCKS
+    proxy.CACHE_CAPACITY_BLOCKS = 2
+    try:
+        block_size = proxy.BLOCK_SIZE
+        a = [1] * block_size
+        b = [2] * block_size
+        c = [3] * block_size
+        inst.record_prefix(a)
+        inst.record_prefix(b)
+        # Touch A so it becomes MRU; B is now LRU.
+        assert inst.estimate_cache_hit(a) == block_size
+        # Insert C: B should be evicted, A should remain.
+        inst.record_prefix(c)
+        assert inst.estimate_cache_hit(a) == block_size
+        assert inst.estimate_cache_hit(b) == 0
+    finally:
+        proxy.CACHE_CAPACITY_BLOCKS = saved
+
+
+def test_pick_instance_session_affinity_sticks(proxy):
+    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
+    affinity = {"sess1": 1}
+    chosen, idx = proxy.pick_instance(insts, None, "sess1", 1000, affinity)
+    assert idx == 1 and chosen is insts[1]
+
+
+def test_pick_instance_session_affinity_breaks_on_overload(proxy):
+    """When the pinned instance is heavily overloaded, fallback to load-aware pick."""
+    insts = [
+        _make_inst(proxy, "http://a", ongoing_tokens=100),
+        _make_inst(proxy, "http://b", ongoing_tokens=1_000_000),
+        _make_inst(proxy, "http://c", ongoing_tokens=100),
+    ]
+    affinity = {"sess1": 1}
+    chosen, idx = proxy.pick_instance(insts, None, "sess1", 1000, affinity)
+    # avg ~333k; B at 1M is ~3x avg, well above OVERLOAD_FACTOR=2.0 -> fallback.
+    assert idx != 1
+    assert chosen is not insts[1]
+
+
+def test_pick_instance_p_offload_penalty_steers_away(proxy):
+    """Instances actively running offloaded HEAVY prefills get penalized."""
+    insts = [
+        _make_inst(proxy, "http://a", ongoing_tokens=0, active_p_offloads=2),
+        _make_inst(proxy, "http://b", ongoing_tokens=1000),
+    ]
+    chosen, idx = proxy.pick_instance(insts, None, None, 5000, {})
+    # B's 1000-token load is much smaller than A's 2 * HEAVY_THRESHOLD penalty.
+    assert idx == 1 and chosen is insts[1]
+
+
+def test_pick_instance_lmetric_picks_lowest_score(proxy):
+    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
+    insts[0].pending_prefill_tokens = 0
+    insts[0].num_requests = 0
+    insts[1].pending_prefill_tokens = 5000
+    insts[1].num_requests = 4
+    chosen, idx = proxy.pick_instance_lmetric(insts, None, None, 1000, {})
+    # Empty instance has score = 1000 * 0 = 0; busy one has (5000+1000)*4.
+    assert idx == 0 and chosen is insts[0]