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

tests/unit/test_formatting.py

@@ -0,0 +1,51 @@
+"""Unit tests for :mod:`otko.views.formatting` (no Qt required)."""
+
+from __future__ import annotations
+
+import pytest
+
+from otko.core import UnitSystem
+from otko.views.formatting import (
+    DOF_LABELS,
+    MASS_LABELS,
+    RESTRAINT_LABELS,
+    format_number,
+    parse_float,
+    parse_int,
+    unit_label,
+)
+
+
+def test_label_tuples() -> None:
+    assert DOF_LABELS == ("Ux", "Uy", "Uz", "Rx", "Ry", "Rz")
+    assert MASS_LABELS == ("Mx", "My", "Mz", "Ixx", "Iyy", "Izz")
+    assert RESTRAINT_LABELS == ("Ux", "Uy", "Uz", "Rx", "Ry", "Rz")
+
+
+def test_format_number() -> None:
+    assert format_number(50.0) == "50"
+    assert format_number(1.5) == "1.5"
+
+
+def test_parse_float_required() -> None:
+    with pytest.raises(ValueError):
+        parse_float("", "x")
+
+
+def test_parse_float_allow_empty() -> None:
+    assert parse_float("", "x", allow_empty=True) == 0.0
+
+
+def test_parse_int_rejects_fractional() -> None:
+    with pytest.raises(ValueError):
+        parse_int("1.5", "x")
+
+
+def test_unit_label() -> None:
+    assert unit_label("force", UnitSystem.SI_M_N) == "N"
+    assert unit_label("moment", UnitSystem.US_FT_KIP) == "kip·ft"
+
+
+def test_unit_label_unknown_quantity() -> None:
+    with pytest.raises(ValueError):
+        unit_label("temperature", UnitSystem.SI_M_N)