Generate Rust bytecode dump helpers from Bytecode.def and route
Executable::dump() through them for instruction stream formatting.
Add a small Rust runtime::value helper for decoding encoded LibJS
Values so immediate Value operands are formatted on the Rust side. C++
callbacks remain only for local names and GC-backed Value payloads that
still need LibJS object access.
Remove the generated C++ to_byte_string_impl() methods and the old
Instruction::to_byte_string() dispatch. The bytecode dump tests cover
output compatibility.
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