fix(codegen): serialize object keys safely

[dajiaohuang]

Fixes #361.

Quote non-identifier object keys when serializing JavaScript object literals while keeping simple identifier keys readable. Emit __proto__ as a computed key so it remains an own data property rather than invoking object-literal prototype syntax.

Adds regression coverage for hyphenated keys, simple keys, and __proto__ preservation.

Validation:

• pnpm exec vitest run packages/codegen/src/serialize.test.ts (3 tests pass)
• pnpm --filter @json-render/codegen run typecheck (pass)
• pnpm --filter @json-render/codegen run build (pass)
• pnpm exec prettier --check packages/codegen/src/serialize.ts packages/codegen/src/serialize.test.ts (pass)
• Node parser/semantics check for hyphenated and __proto__ keys (pass)

The full workspace pnpm type-check was attempted but could not complete on this Windows host: Turbo reaches the unrelated @json-render/next#build script, which uses rm -rf dist and fails because rm is unavailable.

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