Fix recursive aliases with Record references

[xianjianlf2]

Fixes #1640.

When bootstrapping alias references, preserve references already present on the current node instead of overwriting them with same-id references from another scope. This keeps a recursive $Array alias bound to the user scope when another alias references Record<string, Node>.

Tests:

• pnpm testTyped ark/type/__tests__/realWorld.test.ts --grep "cyclic discriminated union with record reference"
• pnpm testTyped ark/type/__tests__/realWorld.test.ts --grep "cyclic discriminated union"
• pnpm testTyped ark/type/__tests__/discrimination.test.ts
• pnpm testTyped ark/schema/__tests__/scope.test.ts ark/type/__tests__/scope.test.ts

[pullfrog]

✅ No new issues found.

Reviewed changes — Replaces Object.assign with a ??=-based merge for alias reference bootstrapping, preventing recursive aliases from being overwritten when a Record<string, Node> reference creates same-id references in a different scope.

• addReferences preserves existing references — Object.assign in bootstrapAliasReferences would overwrite an alias node's scope-bound references with same-id entries from a generic resolution's scope (e.g. Record<string, Node>). The new addReferences helper uses ??= so the existing (scope-correct) reference wins.
• Test: cyclic discriminated union with record reference — covers the exact scenario from #1640 where Node, Array { item: "Node" }, and Container { things: "Record<string, Node>" } are defined together in a scope.

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