Extract shared interface for composite value types

[nathanacurtis]

Problem

Adding a new composite property type (like a new multi-value style) requires changes in many places throughout the codebase. Each new type needs its own handling in style differencing, data output, cloning, merging, and variable lookup — all following the same pattern. This makes the codebase harder to maintain and increases the risk of missing a required change.

Solution

Extract a shared interface for composite values so that adding a new composite type requires implementing the interface in one place rather than updating multiple files with repetitive logic.

Acceptance criteria

• New composite types can be added by implementing a single interface
• Existing composite types (quad and biaxial) continue to work correctly
• No behavioral changes to output — this is a pure refactor

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Implementation details are tracked internally.