Model multiple-inheritance vtbl bases as ABI-correct subobjects

[tannergooding]

A struct deriving from two or more polymorphic bases previously flattened every base into a single lpVtbl, which emitted duplicate members (a CS0111 compile error) and silently dispatched secondary-base methods through the primary vtable. This builds on #748 (the interim dedupe) and makes the layout ABI-correct.

Only the primary (first) polymorphic base shares the derived type's vtable pointer, so it alone is flattened into lpVtbl; every other polymorphic base is emitted as a nested subobject field, preserving its own vtable pointer at the correct offset. A field-bearing polymorphic primary base cannot be flattened and is likewise emitted as a subobject, so the derived type declares no lpVtbl of its own.

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Nested multiple inheritance would require carrying a flattened base's own non-primary polymorphic subobjects onto the derived type. Faithfully modeling that needs changes to the generator's naming and base-field/dispatch resolution (both assume every emitted base is a direct base of the record), so it is deferred. Those records fall back to the legacy run-on lpVtbl -- every inherited method stays reachable so the bindings still compile -- and are flagged with a warning rather than silently mislaid out.

Known limitations (deferred, tracked for follow-up):

• Nested multiple inheritance -> legacy run-on lpVtbl + warning.
• Virtual/diamond inheritance (CXXBaseSpecifier.IsVirtual) is not modeled anywhere in the generator (pre-existing, orthogonal).

Validation:

• Full generator suite green (Failed: 0), zero golden churn.
• Added regression coverage in MultipleBaseVtblSubobjectTest.cs: secondary-base subobject (unifying and non-unifying), explicit-vtbl and marker-interface variants, field-bearing primary, and the nested-MI fallback (asserts the warning).
• Self-hosted bindings contain no multi-base polymorphic structs, so no regeneration is required.

Fixes #592