test(policy): unit tests for Algorithm 1 lex scoring

Adds the project's first test suite. Covers the
score_candidate() pure function from the previous refactor
commit, validating the qualitative properties that
KVC_ROUTER_ALGORITHM.md §3.1 and §4.2 rely on.

Tests / properties:
  - determinism: same args -> same tuple
  - shape: 4-int tuple
  - primary term: overlap dominates pure sticky
  - primary term: sticky_bonus credited
  - tie-2 inflight: lower wins
  - tie-3 assigned: lower wins
  - strict lex order: sticky wins position-1 over fresh-idle
  - load_floor disabled by default
  - load_floor gated off when sticky=True
  - load_floor zero during warmup (mean=0)
  - load_floor proportional to deficit (200/100/0 at 0/50/100% load)
  - load_floor does not underflow when overloaded
  - real per-session overlap beats load_floor on warm D
  - boilerplate overlap loses to load_floor on cold D
    (the cold-D fix from E1_E2_FIX_DESIGN §Q2)

Test infrastructure:
  - tests/ package with README explaining the GPU-free
    scope and the run instruction
  - pyproject.toml [dependency-groups] test = [pytest>=8]
    (install via `uv sync --group test`)
  - pyproject.toml [tool.pytest.ini_options] sets testpaths

Verified locally: 14/14 passing under pytest 9.0.3 in an
isolated 3.13 venv. No SGLang / GPU touched.

tests/README.md

@@ -0,0 +1,39 @@
+# Tests
+
+Pure-Python unit + property tests for the algorithm layer. These tests do
+**not** import SGLang and do **not** need a GPU — they validate the routing
+algorithm (Algorithm 1/2/3 in `docs/KVC_ROUTER_ALGORITHM.md`) and its
+theorems against the pure functions extracted from `policies.py`.
+
+## Run
+
+```bash
+uv sync --group test
+uv run pytest
+```
+
+Or, without uv:
+
+```bash
+pip install pytest
+PYTHONPATH=src pytest tests
+```
+
+## Scope
+
+- `test_policy_scoring.py` — Algorithm 1 lex-score properties (overlap
+  dominates sticky, load-floor gating, tie-breakers).
+- `test_no_starvation.py` — Theorem 1: bounded retries before some D either
+  accepts or the least-rejected D is forced through the degenerate path.
+
+Future:
+- block-level eviction `MockRadixCache` tests (see
+  `docs/BLOCK_LEVEL_EVICTION_DESIGN_ZH.md` §5).
+- D→P sync `staleness_budget` property tests (see
+  `docs/D_TO_P_SYNC_CONTRACT_ZH.md` §1).
+
+## Why no integration tests here
+
+Anything that needs SGLang, mooncake, or a real model is an integration
+test and must run on hardware. Those tests live as `scripts/sweep_*.sh`
+under the evaluation protocol in `docs/EVALUATION_PROTOCOL_ZH.md`.