Cleaned dead code and updated documents

docs/CACHE_KEYING.md

@@ -16,14 +16,22 @@ standardized replacement exists.
 ## Resolution: HMAC cache key (JSON-canonical form)
 
 ```
-HMAC-SHA256(secret, JSON.stringify({
+ctx:{context}:HMAC-SHA256(secret, json.dumps({
     "c": context,
-    "p": sorted_params,
+    "p": sorted_params,   // values normalized to JSON-native strings
     "r": rev,
-    "u": user_id    // omitted for public content
-}, sort_keys=True))
+    "u": user_id          // omitted for public content
+}, sort_keys=True, separators=(",", ":")))
 ```
 
+`derive_cache_key(secret, context, params, user_id=None, rev=0)` →
+`"ctx:{context}:{hmac_hex}"`. The `ctx:{context}:` prefix lets broad
+purge SCAN by prefix. Param values are normalized for cross-language
+consistency (`True`→`"true"`, `None`→`"null"`) before stringification.
+Implemented in `cores/mizan-python/src/mizan_core/cache/keys.py` and
+`backends/mizan-ts/src/cache/keys.ts` (`deriveCacheKey`); pin tests
+verify identical output.
+
 ### Key derivation rules
 
 - **Public content** — URL path + query params (standard CDN).
@@ -45,20 +53,24 @@ Mizan claims on `X-Mizan-Token` header. Replaces the old
 **Not a compiled binary ABI. Not a pluggable Python protocol.**
 
 Each backend adapter (Python, TypeScript, future PHP/C#/Go)
-implements the cache protocol in its own language, backed by Redis.
+implements the cache protocol in its own language.
 **Conformance verified by a shared test suite.**
 
 ### Required operations
 
 - `cache_get`
 - `cache_put`
-- `cache_purge`
-- `cache_purge_user`
+- `cache_purge` (scoped recomputes the key; broad SCANs the
+  `ctx:{context}:*` prefix)
 
 ### Storage
 
-Redis only. Handles persistence, cross-worker sharing, crash
-recovery.
+Two backends behind a `CacheBackend` protocol
+(`mizan_core/cache/backend.py`):
+
+- `MemoryCache` — dict-based, for testing.
+- `RedisCache` — production; persistence, cross-worker sharing, crash
+  recovery. Broad purge via SCAN, delete via UNLINK.
 
 ## Deploy invalidation