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.

pyproject.toml

@@ -20,8 +20,21 @@ build-backend = "setuptools.build_meta"
 [tool.setuptools.packages.find]
 where = ["src"]
 
+[dependency-groups]
+# Pure-Python unit tests. Install via:
+#   uv sync --group test
+# These tests deliberately import only the algorithm-layer modules
+# (policies, trace, topology) so they run without SGLang / GPU / CUDA.
+test = [
+    "pytest>=8.0",
+]
+
 [tool.uv]
 prerelease = "allow"
 
 [tool.uv.sources]
 sglang = { path = "third_party/sglang/python", editable = true }
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+addopts = "-q"