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>

cores/mizan-python/tests/test_keys.py

@@ -0,0 +1,68 @@
+"""Unit tests for cache key derivation. Includes the cross-language pin against mizan-ts."""
+
+from unittest import TestCase
+
+from mizan_core.cache.keys import derive_cache_key
+
+
+class CacheKeyDerivationTests(TestCase):
+    """Tests that HMAC cache key derivation is deterministic and correct."""
+
+    SECRET = "test-cache-secret"
+
+    def test_deterministic_output(self):
+        """Same inputs always produce the same key."""
+        key1 = derive_cache_key(self.SECRET, "user", {"user_id": "5"})
+        key2 = derive_cache_key(self.SECRET, "user", {"user_id": "5"})
+        self.assertEqual(key1, key2)
+        self.assertTrue(key1.startswith("ctx:user:"))
+        self.assertEqual(len(key1), len("ctx:user:") + 64)  # prefix + SHA-256 hex
+
+    def test_param_order_irrelevant(self):
+        """Parameter ordering does not affect the key."""
+        key1 = derive_cache_key(self.SECRET, "ctx", {"a": "1", "b": "2"})
+        key2 = derive_cache_key(self.SECRET, "ctx", {"b": "2", "a": "1"})
+        self.assertEqual(key1, key2)
+
+    def test_different_user_ids_different_keys(self):
+        """Different user_ids produce different cache keys."""
+        key1 = derive_cache_key(self.SECRET, "user", {"user_id": "5"}, user_id="5")
+        key2 = derive_cache_key(self.SECRET, "user", {"user_id": "5"}, user_id="6")
+        self.assertNotEqual(key1, key2)
+
+    def test_rev_changes_key(self):
+        """Different rev values produce different cache keys."""
+        key1 = derive_cache_key(self.SECRET, "user", {"user_id": "5"}, rev=0)
+        key2 = derive_cache_key(self.SECRET, "user", {"user_id": "5"}, rev=1)
+        self.assertNotEqual(key1, key2)
+
+    def test_no_delimiter_collision(self):
+        """JSON-canonical form prevents delimiter-free concatenation collisions."""
+        # "user" + user_id="12" + params="3" vs "user1" + user_id="2" + params="3"
+        key1 = derive_cache_key(self.SECRET, "user", {"id": "3"}, user_id="12")
+        key2 = derive_cache_key(self.SECRET, "user1", {"id": "3"}, user_id="2")
+        self.assertNotEqual(key1, key2)
+
+    def test_public_vs_user_scoped(self):
+        """Public (no user_id) and user-scoped produce different keys."""
+        public = derive_cache_key(self.SECRET, "products", {"id": "1"})
+        scoped = derive_cache_key(self.SECRET, "products", {"id": "1"}, user_id="5")
+        self.assertNotEqual(public, scoped)
+
+    def test_cross_language_pin(self):
+        """Pinned HMAC values — must match TypeScript adapter exactly."""
+        pin_secret = "test-pin-secret-that-is-32bytes!"
+
+        public_key = derive_cache_key(pin_secret, "user", {"user_id": "5"}, rev=0)
+        self.assertEqual(
+            public_key,
+            "ctx:user:605a1ca5ad5994e9b765c8d1b330474c2a0d51a7b8fbbdc402f992da7ba902f6",
+        )
+
+        user_scoped_key = derive_cache_key(
+            pin_secret, "user", {"user_id": "5"}, user_id="5", rev=0,
+        )
+        self.assertEqual(
+            user_scoped_key,
+            "ctx:user:30fc08eb46ee4ff2cf7d317e97dca90fd616511e0587304416f71dc863338dc2",
+        )