Integrate unified PDF modeling backends - #3

What changed

• add backend discovery for PDFgui/PDFfit2, diffpy.structure, SrReal, SrFit, DiffPy-CMI, diffpy.morph, RMCProfile, and fullrmc
• add deterministic workflow planning with bounded AI explanation prompts
• add actual SrReal simulation, SrFit single-phase refinement, and diffpy.morph comparison adapters
• add a shell-free external-process runner for separately installed RMCProfile and fullrmc environments
• add pdfgui-model commands for engine diagnostics, planning, simulation, refinement, comparison, and external runs
• add PDFgui menu entries for engine status, workflow planning, AI explanation, and SrReal simulation
• add optional dependency metadata, a Python 3.13 environment, documentation, release notes, and mocked integration tests

Distribution and compatibility boundaries

• RMCProfile is not bundled; users configure PDFGUI_RMCPROFILE_EXECUTABLE
• fullrmc remains in a separate Python process; users configure PDFGUI_FULLRMC_PYTHON
• current SrReal, SrFit, and DiffPy-CMI distributions require Python earlier than 3.14, so the complete environment pins Python 3.13
• Python 3.14 remains supported by PDFgui; backend discovery reports the incompatible optional packages as unsupported

Review focus

• optional-package imports must remain lazy so base PDFgui startup is unaffected
• external execution must keep shell=False and preserve explicit argument boundaries
• generated AI prompts must omit raw PDF arrays and structure contents and redact local paths
• mocked adapters must follow the published SrReal, SrFit, and diffpy.morph interfaces

Validation

A dedicated tests/test_modeling.py suite covers backend discovery, Python-version gates, workflow selection, AI-prompt bounds, external execution, and mocked SrReal/SrFit/Morph calls. GitHub Actions and final pre-merge checks are pending on this draft.