Prepare DTS work branch

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docs/source/api/dts.rst

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+.. _api_dts:
+
+DTS Interface
+=============
+
+.. currentmodule:: supy.dts
+
+The DTS (Derived Type Structure) interface provides a direct
+Pydantic-to-Fortran execution path for SUEWS. It bypasses the intermediate
+``df_state`` conversion layer used by the traditional backend.
+
+.. note::
+
+   This page is an advanced API reference for developers and maintainers.
+   Standard user workflows remain unchanged and require no DTS-specific steps.
+
+Build Requirements
+------------------
+
+DTS features are available only in a full DTS build:
+
+.. code-block:: bash
+
+   make clean && make dev-dts
+
+Fast builds (``make dev``) do not include DTS type wrappers.
+
+For most users, DTS is an internal execution detail. The public
+``SUEWSSimulation`` workflow remains the recommended interface.
+
+Main DTS Runners
+----------------
+
+.. autosummary::
+    :toctree: _autosummary
+
+    run_dts
+    run_dts_multi
+
+Factory Functions
+-----------------
+
+.. autosummary::
+    :toctree: _autosummary
+
+    create_suews_config
+    create_suews_site
+    create_suews_state
+    create_suews_forcing
+    create_suews_timer
+    create_output_line
+
+Population Functions
+--------------------
+
+.. autosummary::
+    :toctree: _autosummary
+
+    populate_config_from_pydantic
+    populate_site_from_pydantic
+    populate_state_from_pydantic
+    populate_forcing_from_row
+    populate_timer_from_datetime
+    populate_atmstate
+    populate_roughnessstate
+    populate_ohmstate_defaults
+    populate_storedrainprm
+
+Extraction Functions
+--------------------
+
+.. autosummary::
+    :toctree: _autosummary
+
+    extract_output_line_to_dict
+    build_output_dataframe_from_block
+    build_full_output_dataframe
+    extract_state_from_dts
+
+Performance Considerations and Best Practice
+--------------------------------------------
+
+- Use DTS when your workflow is YAML/Pydantic-centred.
+- For multi-grid workloads, set ``n_jobs`` via ``sim.run(backend="dts", n_jobs=...)``.
+- Use ``chunk_day`` for long runs to bound memory usage.
+- Keep forcing validation and output QA checks identical across backends.
+
+FAQ
+---
+
+**Why does DTS fail in a fast build?**
+   DTS requires type wrappers that are not present in ``make dev`` builds.
+
+**Can I save DTS runs with ``sim.save()``?**
+   Not yet. ``sim.save()`` currently requires DataFrame final state from the
+   traditional backend.
+
+**How do I continue a DTS run?**
+   Use ``sim.initial_states_final`` (Pydantic ``InitialStates``) as the state
+   source for the next run.
+
+Related Documentation
+---------------------
+
+- :doc:`simulation` - ``SUEWSSimulation`` API