Baseline spec and gravity check

First release of ShellBench: the thesis building's baseline specification and the gravity check of the anchor frame.

Contents

• docs/baseline_spec.md: every model input for the thesis building, with its source, and the author's decisions on 16 thesis inconsistencies (B1-B16).
• validation/thesis_values/: thesis results typed as validation targets, with flags D1-D15 (D16-D20 proposed).
• src/shellbench/anchor_frame.py: the thesis transverse frame as a 2D elastic OpenSeesPy model under dead and live load.
• src/shellbench/combinations.py: the thesis load combinations (1.3D + 1.5L evaluated; wind and seismic pending).
• src/shellbench/thesis_checks.py: arithmetic checks on the thesis tables.
• results/: model against thesis, with % differences and check runs. Every difference above 10 % has a written reason in docs/anchor_notes.md.

Screening-level, elastic, code-level analysis under the stated assumptions. No claim of seismic safety.

Tests: pytest, 64 passed. Python 3.12, OpenSeesPy 3.8.0.0. Licence: MIT.