fix(sync): support Ref-typed properties

[0hirume]

Adds full two-way sync support for Ref-typed properties (Model.PrimaryPart, Weld.Part0/Part1, ObjectValue.Value, etc.).

• .rbxm/.rbxmx build/read now preserves internal Ref properties instead of dropping them.
• For filesystem-based two-way sync, Ref properties are represented as relative-path strings in .data.json/.model.json, resolved back to real Variant::Ref targets via Tree::resolve_refs().
• Fixes a crash ("Instance meta not found") when inserting .rbxm/.rbxmx files containing internal Ref properties: snapshot_from_dom was reusing Snapshot.id as a remap placeholder, which collided with its other meaning ("corresponds to an existing tree instance") and caused new instances to be misrouted as updates. Introduces a separate transient Snapshot.ref_id field and a two-pass insert + remap, applied to both the live insertion path and the initial tree build path.
• cargo test --lib (8/8 pass)
• cargo clippy --all-targets clean
• Manual end-to-end test via argon serve + companion Studio plugin: created a Model with PrimaryPart and a Weld with Part0/Part1, confirmed .rbxm/.data.json round-trip and live two-way sync work without crashing.

Closes #269