[codex] open-source baseline

[suchipizza]

What changed

This PR turns the repository into a credible open-source baseline for FEA Copilot and closes the remaining repository-level launch work from the PRD.

It adds or tightens the public repo surface:

• refreshed README.md with a validation summary table and clearer doc links
• AGENTS.md, CONTRIBUTING.md, CHANGELOG.md, ROADMAP.md, CODE_OF_CONDUCT.md, SECURITY.md, and CITATION.cff
• issue templates and PR template
• Makefile, .devcontainer/devcontainer.json, OPEN_SOURCE_AUDIT.md, and OPEN_SOURCE_READINESS_REPORT.md
• launch drafts under launch/ and 10 issue drafts under launch/issues/
• repository-settings, assumptions, repo-map, LLM quickstart, and release-checklist docs

It also expands the reproducible engineering surface:

• validation schema and validation runner (tools/run_validation.py)
• a committed public hand-calculation validation case under validation/public_formula_checks/
• solver-comparison intake scaffolding under validation/solver_comparison/
• exporter, example, validation-case, and solver-adapter template coverage
• CI now runs the validation suite in addition to the existing smoke checks

Finally, it includes the visualization/runtime support that was already present locally and now passes tests on-branch:

• persisted result rendering in the Streamlit app
• plotly plus PyVista visualization plumbing in the service and visualizer path
• dependency and tests updates for h5py / pyvista

Why it changed

The repository was already close to open-source ready, but it still lacked several launch-critical pieces from the PRD:

• a standard local command surface (make test, make validate, make example)
• a Codespaces/devcontainer path
• a fourth committed validation case plus validation intake structure
• launch drafts and issue drafts
• explicit audit/readiness reporting
• GitHub metadata and branch/ruleset alignment

This branch finishes those pieces without expanding the solver scope beyond the current supported regime.

Impact

For users:

• clearer scope, assumptions, and validation evidence
• a more complete quickstart and local command surface
• better visualization support for solver-field results

For contributors:

• more obvious entry points, issue drafts, and templates
• validation and benchmark structure that is easier to extend safely
• agent-readable repo orientation and contribution constraints

Validation

Validated locally from this branch state:

• python3 -m py_compile app.py fea_engine/*.py templates/*.py tests/*.py tools/*.py validation/public_formula_checks/*.py validation/mesh_convergence/*.py validation/roark_formulas/*.py
• pytest -q (81 passed, 3 skipped)
• make test
• make validate
• make examples
• python3 tools/check_markdown_links.py

GitHub metadata also updated during this pass:

• repository topics configured
• repository description tightened
• default-branch ruleset corrected to require test and docker-smoke