add test fixtures for arithmetic, bitwise, functions, objects, edge cases

[cs01]

Summary

• adds test fixtures for categories that had 0 TS tests: arithmetic, bitwise, error-handling
• adds tests for thin categories: functions (nested calls, multiple returns), objects (nested access), edge-cases (empty string ops, zero/negative numbers)
• bitwise and try-catch marked @test-skip (not yet supported)

Test plan

• verify:quick passes (tests + stage 1 self-hosting)

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