feat(phase-14a): Stubs, bitwise fixes, and math return types

[davydog187]

Summary

• collectgarbage stub accepting all standard modes with plausible return values
• dofile stub raising "not supported in embedded mode"
• _VERSION global set to "Lua 5.3"
• Global unpack alias for table.unpack
• Bitwise string coercion (e.g., "0xff" | 0)
• Lua 5.3 shift semantics: negative shift reverses direction, shift >= 64 yields 0
• Bitwise metamethods: __band, __bor, __bxor, __bnot, __shl, __shr
• math.floor/math.ceil return integers instead of floats

Test plan

• collectgarbage returns without error (2 tests)
• _VERSION == "Lua 5.3" (1 test)
• unpack({10, 20, 30}) works as global alias (1 test)
• Bitwise string coercion and shift edge cases (3 tests)
• math.floor/ceil return integers (2 tests)
• Updated math_test.exs assertions for integer return types
• 1224 tests, 0 failures, 35 skipped

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