Add native furniture variant cloning

[diffuin]

Requested by @ifBars in #236.

Diffuin response

Confidence: Medium

Implemented native furniture variant cloning. The additive API safely clones presentation-only grid or surface donors, preserves donor placement and item defaults, isolates materials, and routes the customized visual through placed, stored, ghost, and generated-icon representations. Repository compilation remains unverified because the checkout environment has no .NET SDK.

Recommended next steps

1. Callbacks run once against an unparented, builder-owned visual clone whose renderer materials are already independent from native assets.
2. Placed, stored, ghost, and icon visuals each receive separate material instances.
3. Existing CreateBuilder and WithModel behavior remains unchanged; material isolation is enabled only for native variants.

Evidence and validation

Evidence inspected

• Added FurnitureCreator.CloneFrom(string) and CloneFrom(BuildableItemDefinition), plus ConfigureModel(Action<GameObject>).
• Donors using specialized GridItem or SurfaceItem subclasses are rejected; non-presentation components are removed before the callback receives the clone.
• Exact donor grid coordinates or surface settings are preserved by default, alongside build sound, pricing, stack limit, and donor icon fallback.
• Added API compatibility, compile-fixture, clone-policy, copied-default, override, and sound-mapping coverage, plus native-variant documentation.

Validation performed

• Read the repository standards and required workflow skills.
• Verified relevant regular and beta stripped Mono declarations match and inspected specialized buildable subclasses.
• Read issue [FEATURE] Support customizable variants of native furniture #236 and prior [FEATURE] Fully support custom placeable furniture #232/feat(items): add custom furniture API #233 context through the read-only GitHub broker; searched the private AssetRipper corpus for furniture presentation evidence.
• Ran git diff --check successfully and completed a static public-API and call-site audit.

Runtime validation remaining

• Run MonoMelon and Il2CppMelon restore, solution build, and test commands. dotnet is not installed in this environment.
• Run DocFX, documentation coverage, and ApiCompat against the exact target assembly.
• Validate representative grid and surface donors in both game backends, including material isolation, placement, storage, generated icons, save restoration, multiplayer placement, and late join.

Diffuin run details

• Provider: codex
• Model: gpt-5.6-sol
• Reasoning: high (Luna advisor: risk (high); baseline bounded implementation)
• Elapsed: 1162s
• Codex thread: 019ff7b6-c147-7c33-85c3-f81baafd81d7

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AI notice: Generated with AI assistance and not guaranteed accurate. Verify findings and plans against the current source and runtime.

Closes #236

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Diffuin job: 18fb5df1-6b62-48bc-be63-406ffd49c67e
Commit: 5bf52188959cde3a8d89133fb036fd239aa24f98