Extract Layer 1 to cores/mizan-python (mizan-core)

Pull cache/keys.py (HMAC cache key derivation) and mwt.py (Mizan Web Token)
out of backends/mizan-django and into a new cores/mizan-python package.
mizan-django re-imports them via the new mizan_core module.

Naming: directory cores/mizan-python/, distribution mizan-core, importable
module mizan_core. mizan-django keeps its existing 'mizan' distribution slot
on PyPI; the two coexist as distinct packages.

Wiring:
- backends/mizan-django/pyproject.toml gains a 'mizan-core' dep with a
  [tool.uv.sources] path entry (editable install from ../../cores/mizan-python).
- Makefile install target prepends 'cd cores/mizan-python && uv pip install -e .'
- 3 import sites in mizan-django updated: cache/__init__.py, jwt/functions.py,
  client/executor.py — all now import from mizan_core.

Test split:
- 3 unit-test classes (CacheKeyDerivationTests, MWTCreationTests,
  PermissionKeyTests) move to cores/mizan-python/tests/, rewritten against
  unittest.TestCase (no Django dep). The cross-language pin test (pinned
  HMAC hex digests against mizan-ts) moves with CacheKeyDerivationTests.
- Integration tests stay in mizan-django (CacheBackendTests, CachePurgeTests,
  CacheIntegrationTests, RevParameterTests, MWTAuthIntegrationTests) — they
  need the Django request flow.

Verified:
- mizan-core: 15/15 pass (incl. cross-language pin)
- mizan-django: 348 pass, 21 skip, 0 fail
- mizan-ts: edge-compat 34/34 pass — protocol invariant holds, the moved
  Python derive_cache_key still produces the exact hex digests TS pins against.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backends/mizan-django/pyproject.toml

@@ -5,6 +5,7 @@ description = "Django + React server functions framework"
 readme = "README.md"
 requires-python = ">=3.10"
 dependencies = [
+    "mizan-core",
     "django>=5.0",
     "django-ninja>=1.0",
     "django-readers>=2.0",
@@ -12,6 +13,9 @@ dependencies = [
     "PyJWT>=2.0",
 ]
 
+[tool.uv.sources]
+mizan-core = { path = "../../cores/mizan-python", editable = true }
+
 [project.optional-dependencies]
 cache = [
     "redis>=5.0",