Implement transactional frame entry/leave, right-to-left arguments, one- and
two-byte returns, and guards for both the software stack and the 6502 hardware
stack. Add a recursive x128 ABI fixture covering bounded success and overflow
unwind.

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Emit a BASIC-stub PRG with ordered sections and a MAP entry/section table,
refuse existing output names, and scratch partial files on failure. Include
a runnable startup that reports through the test mailbox.

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