Extract shared utilities from @refrakt-md/svelte to @refrakt-md/transform

Prerequisite for all framework adapters. Two modules in @refrakt-md/svelte are framework-agnostic and needed by every adapter package.

Acceptance Criteria

serialize() and serializeTree() moved to packages/transform/src/serialize.ts
matchRouteRule() moved to packages/transform/src/route-rules.ts
Both are exported from @refrakt-md/transform public API (packages/transform/src/index.ts)
@refrakt-md/svelte re-exports both modules — no breaking change for existing consumers
@refrakt-md/transform adds @markdoc/markdoc as a dependency (needed by serialize())
All existing tests pass without modification
Build succeeds in dependency order (types → transform → svelte)

Copy packages/svelte/src/serialize.ts → packages/transform/src/serialize.ts
Copy packages/svelte/src/route-rules.ts → packages/transform/src/route-rules.ts
Update imports in both files if needed (type imports should already point at @refrakt-md/types)
Export from packages/transform/src/index.ts
Replace original files in packages/svelte/src/ with re-exports from @refrakt-md/transform
Update any internal imports within packages/svelte/ that reference these modules directly