feat: GUI overhaul, solver refactor, Windows build, README update

[test1card]

1. GUI (src/venting/gui/app.py):

   • Fix critical _read_cfg bug: use dataclasses.fields() for type
     inference instead of hardcoded exclusion set — fixes ValueError
     on topology="two_chain_shared_vest"
   • Group 40+ flat fields into 6 QGroupBox sections (topology,
     internal orifices, exit orifice, external/profile, thermodynamics,
     calculation params)
   • Add profile_file Browse button + drag-and-drop CSV support
   • Dynamic field visibility: tube params hide for orifice model,
     pump fields show/hide with external_model, etc.
   • QProgressBar in toolbar with live progress % during solve
   • Multi-node plots with NODE_COLORS (up to 8 nodes per graph)
   • Legends on all plots; P_ext shown as dashed line
   • Color-coded validity table (ok=green, warning=yellow, fail=red)
   • Run history (last 10 runs) with double-click restore
   • Tooltips with units/ranges on all numeric fields

2. Solver refactor (src/venting/solver.py):

   • Extract ~120 lines of duplicated setup into _prepare_solve()
   • solve_case and solve_case_stream now delegate common setup
     (dynamic_pump rewiring, y0 build, rhs+pump closure, max_step)
     to _prepare_solve; no behavior change, all tests pass

3. Windows build:

   • Add venting.spec (PyInstaller, single-file exe, no console window)
   • Add build_windows.py (auto-detects UPX, cleans dist/build)
   • Add .github/workflows/build_windows.yml (builds on tag push,
     uploads artifact, creates GitHub Release)
   • Add pyinstaller>=6.0 to dev deps; update gui dep to PySide6>=6.5

4. README.md:

   • Fix false Fanno claim: document full Fanno-flow implementation
   • Add thermo="variable" with NASA-7 polynomial reference
   • Add all missing modules: thermo, montecarlo, validity, compare,
     presets, cases, gui/ subpackage
   • Add mc and compare CLI commands with examples
   • Fix sparsity claim: Radau uses automatic numerical Jacobian
   • Add GUI capabilities: fanno model, two_chain topology, progress
     bar, multi-node plots, validity colors, history, drag-and-drop
   • Remove TODO packaging placeholder; add Windows exe build docs
   • Add "Известные ограничения" section with honest limitations

https://claude.ai/code/session_01TPJHy2FrVy73zbMxCLSW97

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