sched: add typed occurrence expression kind for schedule inputs (T04)

src/wf_core/models/input_bindings.py

@@ -103,6 +103,46 @@ type InputExpression = Annotated[
 ]
 
 
+class OccurrenceExpression(BaseModel):
+    """Reference one typed schedule-occurrence field.
+
+    Schedule-only leaf: graph expressions must not accept this kind, and
+    ``GraphSourcePath`` roots stay closed to input/state/context.
+    """
+
+    model_config = ConfigDict(extra="forbid", strict=True)
+
+    kind: Literal["occurrence"]
+    field: Literal["schedule_id", "occurrence_id", "scheduled_at"]
+
+
+class ScheduleArrayExpression(BaseModel):
+    """Resolve ordered child schedule expressions into one JSON array."""
+
+    model_config = ConfigDict(extra="forbid")
+
+    kind: Literal["array"]
+    items: list[ScheduleExpression]
+
+
+class ScheduleObjectExpression(BaseModel):
+    """Resolve named child schedule expressions into one JSON object."""
+
+    model_config = ConfigDict(extra="forbid")
+
+    kind: Literal["object"]
+    fields: dict[str, ScheduleExpression]
+
+
+type ScheduleExpression = Annotated[
+    LiteralExpression
+    | OccurrenceExpression
+    | ScheduleArrayExpression
+    | ScheduleObjectExpression,
+    Field(discriminator="kind"),
+]
+
+
 def _raise_limit(limit: str, location: str) -> None:
     raise ValueError(f"input expression {limit} limit exceeded at {location}")
 
@@ -206,5 +246,35 @@ StepInputBinding = TypeAliasType(
 )
 
 
-for _model in (ArrayExpression, ObjectExpression, InputExpressionBinding):
+class ScheduleInputBinding(BaseModel):
+    """Assign one schedule-side expression to a workflow-input target.
+
+    Schedule expressions reuse literal/object/array composition and the same
+    budget validator, but leaves are typed occurrence references only. Graph
+    paths are invalid here.
+    """
+
+    model_config = ConfigDict(extra="forbid")
+
+    target: LocalPath
+    expression: ScheduleExpression
+
+    @field_validator("expression", mode="before")
+    @classmethod
+    def check_limits(cls, value: object) -> object:
+        raw_value = (
+            value.model_dump(mode="python") if isinstance(value, BaseModel) else value
+        )
+        validate_input_expression_limits(raw_value)
+        return value
+
+
+for _model in (
+    ArrayExpression,
+    ObjectExpression,
+    InputExpressionBinding,
+    ScheduleArrayExpression,
+    ScheduleObjectExpression,
+    ScheduleInputBinding,
+):
     _model.model_rebuild()