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

.github/pull_request_template.md

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+## Summary
+
+<!-- One sentence: what does this change? -->
+
+## Linked issue
+
+Closes #
+
+## Type of change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Refactor / cleanup
+- [ ] Documentation
+- [ ] CI / tooling
+
+## Architectural checklist
+
+- [ ] No `from PySide6` in `core/` or `services/`
+- [ ] No `import openseespy` in `core/` or `views/`
+- [ ] Public functions have type hints + docstrings
+- [ ] Heavy work runs off the GUI thread
+
+## Testing
+
+<!-- How was this verified? Reference tests added or analytical checks. -->