feat(structure-editor): union, nested, pointer, bitfields in SimpleBlockInterpreter

[abbaye]

Context

SimpleBlockInterpreter previously skipped union, nested, pointer, and bitfield block types. These are now fully implemented (commit 27ef321a).

What changed

• union: resolves discriminant variable, selects matching variant, executes variant sub-fields or decodes inline ValueType; falls back to first variant
• nested: expands inline struct Fields list; triggers bitfield expansion when Bitfields list is present
• pointer: reads target address at offset, stores in TargetVar, labels result
• bitfields: inline extraction from any field/nested block with Bitfields list; applies ValueMap lookup, stores extracted values as variables

Files affected

• Services/SimpleBlockInterpreter.cs (+223 lines)

Follow-up

• Add unit tests for each new block type
• Handle recursive nested depth > 3 (stack overflow guard)
• Document .whfmt schema additions for union/bitfields