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

specifications/README.md

@@ -0,0 +1,37 @@
+# OTKO — specifications (this repo)
+
+This folder holds **OTKO's own planning documents**. It is deliberately
+separate from the greenfield rebuild spec set at
+`~/Sync/otko-development/specifications` (docs `00`–`14`), which stays the
+reference for *what a conformant rebuild looks like*. Requirement IDs of
+the form `[AREA-NNN]` cited in this folder refer to that rebuild set.
+
+## Why this folder exists
+
+`~/Sync/otko` (this repo) is the **working, feature-rich app** and is the
+primary deliverable. `~/Sync/otko-development` is a **greenfield rebuild**
+that conforms closely to the rebuild specs but is less capable in several
+areas (transient/pushover/response-spectrum, richer element/material
+coverage, `.osmodel` corpus, HDF5). The goal of the plan below is to
+**harvest the rebuild's good, spec-conformant capabilities into this app
+without breaking what already works** — not to rewrite this app to the
+rebuild's architecture.
+
+## Documents
+
+| # | Document | Purpose |
+|---|---|---|
+| 15 | [15-rebuild-adoption-plan.md](15-rebuild-adoption-plan.md) | Dependency-ordered plan for porting rebuild capabilities into this repo, with adoption matrix, phases, gates, risks and explicit skips. |
+
+## Ground rules (summary — see doc 15 §3)
+
+- **Additive, never replacement.** This repo is a superset of the rebuild
+  in the solver/runner/export surface; protect that.
+- **Keep the working app working:** `.osmodel` format, AGPL-3.0 license,
+  Python 3.10–3.12 support, existing tests green.
+- **Adapt, don't copy:** re-type ported code to this repo's APIs
+  (`StaticResults`/`ModalResults`, mutable `Project`, 4-value `UnitSystem`).
+- **One phase = one branch = independently shippable**, each with its own
+  tests and gate.
+- **License hygiene:** the rebuild is MIT, this repo is AGPL-3.0; MIT→AGPL
+  is compatible, but ported files need attribution (see doc 15 §7).