fix json.stringify for inline object literals with booleans and numbers

[cs01]

fix json.stringify for inline object literals

Summary

• JSON.stringify({ ... }) with inline object literals previously fell through to unhandled codegen paths, producing incorrect output for boolean and numeric fields
• Added stringifyObjectLiteral in src/codegen/stdlib/json.ts that walks the ObjectNode AST and builds a yyjson document directly, handling strings, numbers, booleans, and nested objects
• Added csyyjson_obj_add_obj C bridge function to create and attach child objects to a parent during stringification, enabling recursive nested object support
• Declared the new extern in src/codegen/runtime/runtime.ts

Files Changed

File	Change
c_bridges/yyjson-bridge.c	add csyyjson_obj_add_obj for nested object construction
src/codegen/runtime/runtime.ts	declare @csyyjson_obj_add_obj extern
src/codegen/stdlib/json.ts	add stringifyObjectLiteral + buildJsonProperties (recursive)
tests/fixtures/builtins/json-stringify-inline-object.ts	test: mixed string/bool/number fields
tests/fixtures/builtins/json-stringify-inline-bool.ts	test: boolean-only fields
tests/fixtures/builtins/json-stringify-deep-object.ts	test: deeply nested objects
tests/fixtures/builtins/json-stringify-spaces.ts	test: spaces=2 and spaces=4 pretty-printing