fix: remove unnecessary double casts in json, json-array, and variable-allocator

[cs01]

Summary

• Replaced 9 as unknown as double casts with direct as casts in json.ts, json-array.ts, and variable-allocator.ts
• All casts are from Expression union type to its member types (ObjectNode, ArrayNode, TypeAssertionNode, VariableNode) — TypeScript allows direct narrowing casts on union members
• Double casts bypass TypeScript's type checker and give the native compiler less type information for GEP index resolution

Test plan

• tsc --noEmit passes (direct casts are type-safe)
• npm run verify:quick passes (tests + self-hosting Stage 0 + Stage 1)

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