[D13] README Update + Clean Install + Tag v1.0

[Siddardth7]

Due: April 8 | Day 13 of 14

• Update README.md — add CLT vs FEA accuracy table (deflection error, σₓₓ error)
• Add CalculiX installation instructions to README
• Add usage section: python compare_clt_fea.py, jupyter notebook validation.ipynb
• Add SA spot-check results summary
• Test clean install: fresh Python 3.11 venv → pip install -r requirements.txt → python src/main.py
• Push final commit, tag v1.0-complete

✅ Done when: README has accuracy table. Fresh venv install works. v1.0-complete tag pushed.