feat: ForceStreamSession 覆盖 field 与 FN-only 标定（T06b）

新增 measurement_requirements，告知调用方 field 模型需要同帧场数据。补充标量、field、FN-only 三类标定与旧 ForceSession 的数值等价测试，以及标定与帧不匹配的情况：field 缺 context 时构造失败，基线帧缺 field 时零点失败，ready 后缺 field 的帧单独标为 invalid_input。与改动前的旧会话基线逐位一致。回归 381 项、lint 通过。

src/orisys/force_stream_session.py

@@ -194,6 +194,17 @@ class ForceStreamSession:
     def last_output(self) -> ForceStreamOutput | None:
         return self._last_output
 
+    @property
+    def measurement_requirements(self) -> tuple[bool, bool]:
+        """``(want_field, want_contact)`` the installed calibration model asks for.
+
+        A field model (``hhd_normal_l1_field/1``) asks for the same-frame field, so
+        a T05 caller must include ``field`` in the ``ImageMeasurement``; a
+        scalar/FN-only model asks for neither. The engine's answer is reused
+        instead of a second interpretation of the model.
+        """
+        return self._engine.measurement_requirements
+
     @property
     def reference_generation(self) -> int | None:
         """The reference generation this session currently accepts frames for."""