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/logo.svg

@@ -0,0 +1,46 @@
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+  <title>OTKO</title>
+  <desc>Wordmark logo for OTKO — a SAP2000-style desktop GUI for OpenSeesPy.</desc>
+
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+
+  <!-- Background panel — keeps contrast consistent on light and dark GitHub themes -->
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+  <!-- Mark: portal frame with a deformed-shape ghost -->
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+    <text x="202" y="124" font-size="16" font-weight="500" letter-spacing="3" fill="#8FA2BF">
+      A SAP2000-STYLE GUI FOR OPENSEESPY
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