TASK-051-003: Update All using Directives (RFC9111 → Caching)

All consumer files confirmed importing `TurboHttp.Protocol.Caching`; no
`using TurboHttp.Protocol.RFC9111;` remains in the solution. Updates were
applied as part of TASK-051-002 (the namespace rename step).

Author: st0o0

.maggus/features/feature_051.md .maggus/features/feature_051_completed.md.maggus/features/feature_051.md renamed to .maggus/features/feature_051_completed.md

@@ -94,8 +94,8 @@ and their namespace declarations updated.
 **Parallel:** no
 
 **Acceptance Criteria:**
-- [ ] No `using TurboHttp.Protocol.RFC9111;` anywhere in the solution
-- [ ] All identified consumer files import `TurboHttp.Protocol.Caching`
-- [ ] No logic changes — only `using` directive updates
+- [x] No `using TurboHttp.Protocol.RFC9111;` anywhere in the solution
+- [x] All identified consumer files import `TurboHttp.Protocol.Caching`
+- [x] No logic changes — only `using` directive updates
 
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