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

tools/gidopensees_import/schema_model.py

@@ -0,0 +1,74 @@
+"""
+Pydantic v2 intermediate representation for GiD schema files (.mat / .cnd).
+
+Each parsed MATERIAL or CONDITION block becomes an EntrySpec.  Fields within a
+block become FieldSpec objects.  DEPENDENCIES lines are normalised into
+DependencyRule / DependencyAction objects.  The final output is a CatalogSpec
+(serialisable to JSON via model_dump_json()).
+"""
+
+from __future__ import annotations
+
+from typing import Literal
+
+from pydantic import BaseModel, Field
+
+
+class DependencyAction(BaseModel):
+    """One action inside a DEPENDENCIES rule: RESTORE, HIDE, or SET a field."""
+
+    action: Literal["RESTORE", "HIDE", "SET"]
+    field: str
+    target: str  # "#CURRENT#" or a literal value
+
+
+class DependencyRule(BaseModel):
+    """A complete DEPENDENCIES clause: when trigger matches, apply actions."""
+
+    trigger: str
+    actions: list[DependencyAction]
+
+
+class FieldSpec(BaseModel):
+    """One QUESTION block inside a MATERIAL or CONDITION entry."""
+
+    name: str
+    widget_type: Literal["CB", "UNITS", "MAT", "SCALAR", "TUPLE"]
+    options: list[str] = Field(default_factory=list)
+    default: str = ""
+    width_hint: int | None = None
+    help_text: str = ""
+    image: str = ""
+    state: str | None = None  # "HIDDEN" | "DISABLED" | None
+    tkwidgets: list[str] = Field(default_factory=list)
+    dependencies: list[DependencyRule] = Field(default_factory=list)
+    section_title: str | None = None  # enclosing TITLE: context
+
+
+class EntrySpec(BaseModel):
+    """One MATERIAL or CONDITION block."""
+
+    name: str
+    book: str
+    source_type: Literal["MATERIAL", "CONDITION"]
+    comment: str = ""
+    image: str = ""
+    tkwidgets: list[str] = Field(default_factory=list)
+    fields: list[FieldSpec] = Field(default_factory=list)
+    condtype: str = ""       # CONDITION-only: CONDTYPE value
+    condmeshtype: str = ""   # CONDITION-only: CONDMESHTYPE value
+
+
+class BookSpec(BaseModel):
+    """A BOOK section grouping related entries."""
+
+    name: str
+    source: Literal["mat", "cnd"]
+    entries: list[EntrySpec] = Field(default_factory=list)
+
+
+class CatalogSpec(BaseModel):
+    """Top-level output: all books from the .mat and .cnd files."""
+
+    mat_books: list[BookSpec] = Field(default_factory=list)
+    cnd_books: list[BookSpec] = Field(default_factory=list)