feat: named case-result load combinations with full GUI support

Snapshots the current development tree, headlined by proper load
combinations (user request): a reusable LoadCombination entity of
weighted completed static-case results (e.g. 1.2xDead + 1.6xLive).

- core: LoadCombination/LoadCombinationItem entities, Project
  integration (lookup, unique ids, reference validation)
- services: combinations.py (linear superposition + envelope),
  exported via services __init__
- commands: undoable Add/Delete/Update for combinations
- GUI: Load Combinations manager dialog, Run-dialog evaluation,
  envelope display in Results panel, Combinations tab in Table dock
- tests: unit coverage (validation, math, error paths) + integration
  superposition check vs a single factored run

docs/README.md

@@ -0,0 +1,26 @@
+# OTKO Documentation
+
+Index of the project documentation. Start with the quick guide if you
+just want to build and run a model; read the architecture page if you are
+changing code.
+
+| Document | What it covers |
+| --- | --- |
+| [QUICK_GUIDE.md](QUICK_GUIDE.md) | Task-first walkthrough: cantilever model, modal analysis, report and script export, display units, undo/redo. |
+| [architecture.md](architecture.md) | MVVM layering, package responsibilities, threading, persistence, and the fixed OpenSeesPy command order. |
+| [roadmap.md](roadmap.md) | Phase-by-phase feature status, from scaffolding through the earthquake-engineering primitives. |
+| [adr/](adr/) | Architecture Decision Records — the "why" behind individual design choices. |
+| [screenshots/](screenshots/) | Screenshots referenced by the docs and README. |
+
+## Architecture Decision Records
+
+- [ADR-0001 — GiD/OpenSees schema import](adr/ADR-0001-gidopensees-schema-import.md)
+- [ADR-0002 — Headless / GUI dependency split](adr/ADR-0002-headless-gui-dep-split.md)
+
+## Related documentation
+
+- [`../README.md`](../README.md) — project overview, install, and quick start.
+- [`../CONTRIBUTING.md`](../CONTRIBUTING.md) — dev setup, layering rules, commit style, and verify commands.
+- [`../AGENTS.md`](../AGENTS.md) — condensed context for automated agents.
+- [`gap-analysis-gidopensees.md`](gap-analysis-gidopensees.md) — gap analysis against the GiD/OpenSees reference.
+- [`../examples/README.md`](../examples/README.md) — the bundled example models.