fix(core): make stream_validate backend/ctx keyword-only

Prevents positional confusion and makes future parameter additions
to the signature non-breaking for existing subclass overrides.

Assisted-by: Claude Code

Author: planetf1

mellea/core/requirement.py

@@ -284,7 +284,7 @@ async def validate(
             )
 
     async def stream_validate(
-        self, chunk: str, backend: Backend, ctx: Context
+        self, chunk: str, *, backend: Backend, ctx: Context
     ) -> PartialValidationResult:
         """Hook for per-chunk streaming validation.
 

test/core/test_stream_validate.py

@@ -32,7 +32,7 @@ def test_stream_validate_is_coroutine():
 async def test_subclass_can_return_pass():
     class PassRequirement(Requirement):
         async def stream_validate(
-            self, chunk: str, backend: Backend, ctx: Context
+            self, chunk: str, *, backend: Backend, ctx: Context
         ) -> PartialValidationResult:
             return PartialValidationResult("pass")
 
@@ -45,7 +45,7 @@ async def stream_validate(
 async def test_subclass_can_return_fail():
     class FailRequirement(Requirement):
         async def stream_validate(
-            self, chunk: str, backend: Backend, ctx: Context
+            self, chunk: str, *, backend: Backend, ctx: Context
         ) -> PartialValidationResult:
             if "bad" in chunk:
                 return PartialValidationResult("fail", reason="bad word detected")