refactor: clean up bytecode module naming and structure

[zharinov]

Summary

• Remove decorative // ==== separators from format.rs
• Extract ir.rs tests to ir_tests.rs (consistent with project pattern)
• Rename Instruction → InstructionIR (disambiguate from decoded Instruction)
• Rename QTypeId → TypeId (cleaner name, no std::any::TypeId conflict)
• Make ir module private, re-export through mod.rs (facade pattern)

Why

Enforce mod.rs as the single facade for bytecode imports. External code now uses crate::bytecode::* instead of reaching into crate::bytecode::ir::*.