made the docstrings more obvious about the immutability of FitRun

src/easyreflectometry/fitting.py

@@ -487,7 +487,10 @@ class FitRun:
       zero). The per-dataset objective values add up to the pooled one: the
       engine's per-dataset results are disjoint slices of the pooled residuals.
 
-    A record is history: read it, do not write to its dictionaries.
+    A record is history: read it, do not write to it. Being frozen, it
+    protects its fields from reassignment, but not the ``pooled`` and
+    ``per_dataset`` dictionaries inside them, which are plain and shared with
+    every holder of the record. Copy them before changing anything.
     """
 
     status: str

src/easyreflectometry/project.py

@@ -657,7 +657,12 @@ def record_fit(self, prepared: PreparedFit, results: Optional[list] = None, stat
 
     @property
     def last_fit(self) -> Optional[FitRun]:
-        """The record of the latest fit recorded with :meth:`record_fit`, or None."""
+        """The record of the latest fit recorded with :meth:`record_fit`, or None.
+
+        The record is read-only history. Its ``pooled`` and ``per_dataset``
+        dictionaries are shared with the record itself, so copy them before
+        changing anything; see :class:`~easyreflectometry.fitting.FitRun`.
+        """
         return self._last_fit
 
     # ----- structural parameter paths -----