Clarify loading histories and relative metrics

Add undergraduate-friendly loading-history rationale, recommended scale-free metrics, fixed reference denominators, and the MAPE caveat. Data and splits are unchanged.

README.md

@@ -43,6 +43,58 @@ Current trajectory accounting:
 | Prospective sealed test | 128 | Published after model freeze; never used for training or selection |
 | **Total** | **3,788** | Complete histories, not exchangeable independent experiments |
 
+## What is a loading history, and why does it matter?
+
+A loading history is the ordered sequence of deformation applied to a material.
+For an elastic spring, only the present deformation matters. For an
+elastoplastic metal, the route matters as well: loading, unloading, reversing,
+rotating the loading direction or adding a mean strain changes the material's
+internal state. Two specimens can therefore arrive at the same current strain
+with different stresses because they arrived there by different routes.
+
+This dataset varies the history deliberately rather than merely sampling more
+points on one curve:
+
+| History variation | Question being tested |
+|---|---|
+| Proportional and reversed cycles | Can the model reproduce yielding, unloading and the Bauschinger effect? |
+| Non-proportional and rotating multiaxial paths | Can it track changing stress directions and coupled material memory? |
+| Nested minor loops and non-periodic sequences | Can it remember partial unload/reload events instead of resetting its state? |
+| Amplitude and mean shifts | Can it extrapolate when the load becomes larger or biased to one side? |
+| 100--200-cycle histories | Does a small state error accumulate into long-term drift? |
+| Matched coarse/fine discretizations | Is the prediction tied to the physical path or to a particular step size? |
+| Material-parameter variation | Can one conditioned model represent a family of materials rather than one fixed curve? |
+
+The scientific object is therefore not an isolated stress value. It is the
+mapping from **material parameters + ordered loading history + current internal
+state** to the next stress and state. Randomly shuffling time steps or mixing
+related resolution groups across data roles would destroy this meaning.
+
+## Recommended evaluation metrics
+
+Report absolute stress RMSE in MPa together with a scale-free metric. MPa is
+needed for engineering interpretation; a relative metric makes results easier
+to compare across cohorts and stress levels.
+
+- **Relative L2 error (%)** = `||stress_pred - stress_ref||2 / ||stress_ref||2 × 100`.
+  This is the preferred scale-free trajectory/state metric for signed cyclic
+  stresses.
+- **Yield-normalized RMSE (%)** = `RMSE / reference_yield_stress × 100`.
+  This answers how large the average error is relative to the 280 MPa reference
+  yield stress used by the sealed protocol.
+- **R2** is useful as a supplementary global fit indicator, but should not
+  replace an engineering error in MPa.
+- **MAPE is not recommended** because cyclic stress repeatedly crosses zero;
+  division by values near zero can make an accurate prediction look arbitrarily
+  poor.
+
+For the sealed reference data, the stress RMS scales are 359.540 MPa for global
+path OOD, 91.411 MPa for 150/200-cycle histories, 371.687 MPa for amplitude and
+mean shifts, and 206.814 MPa over all 128 trajectories. These denominators are
+published so every model can be compared under the same convention. Model
+scores remain in the model repository to keep this page focused on the data and
+evaluation contract.
+
 ## Releases
 
 - **T2 v1:** 1,008 proportional cyclic J2 trajectories for loading-memory