docs: jot down schema migration rules

packages/excalidraw/data/SCHEMA_MIGRATIONS.md

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+# Schema Migration Guidelines
+
+## Contract
+
+Each migration entry must declare:
+- `version`: integer schema version (monotonic)
+- `title`: short label
+- `description`: required plain-language explanation of what changed
+- `scope`: one or more scopes (`scene`, `library`, `clipboard`, `api`)
+- `apply`: pure element transform
+
+## Versioning Rules
+
+- Use integer versions only (`2`, `3`, `4`, ...).
+- `SCHEMA_VERSIONS.latest` must match the latest migration version.
+- Add a new migration when behavior changes.
+
+## Scope Rules
+
+- Prefer explicit scopes.
+- Use `ALL_SCOPES` only when the same transform is required for every boundary.
+- If a migration was missing a scope but transform is unchanged, widening scope is allowed.
+
+## Field Stability
+
+Migrations must only rely on stable, persisted schema fields.
+Do not read or write temporary/PR-only fields.