TASK-050-003: Update all using directives from RFC9110 to Semantics

All `using TurboHttp.Protocol.RFC9110;` directives were already replaced by
TASK-050-002 as part of the namespace rename. Verified zero remaining occurrences
and confirmed build succeeds with 0 errors.

Author: st0o0

.maggus/features/feature_050.md .maggus/features/feature_050_completed.md.maggus/features/feature_050.md renamed to .maggus/features/feature_050_completed.md

@@ -101,8 +101,8 @@ and their namespace declarations updated.
 **Parallel:** no
 
 **Acceptance Criteria:**
-- [ ] No `using TurboHttp.Protocol.RFC9110;` anywhere in the solution
-- [ ] All identified consumer files import `TurboHttp.Protocol.Semantics`
-- [ ] No logic changes — only `using` directive updates
+- [x] No `using TurboHttp.Protocol.RFC9110;` anywhere in the solution
+- [x] All identified consumer files import `TurboHttp.Protocol.Semantics`
+- [x] No logic changes — only `using` directive updates
 
 ---