refactor: classify SysV call args with one slot walker

[rieske]

Summary

• Defer SysV GP/xmm/stack packing until emit: procedureArgument records source order; emitCallArguments classifies.
• One walker (takeSysVArgSlot) is shared by caller emit and callee startProcedure. Floats do not consume GP slots; overflow is source order then right-to-left push.
• Slot-sequence unit tests plus emit goldens for movd, AL, and odd-stack pad. Functional mixed printf("%.1f %d") and combine(float, int).

Test plan

• Local codegenTest green (including SysVCallConv)
• Local float/call functional suite (intel) green
• CI build workflow (build + serial tests + coverage / Coveralls)

Coveralls: 92.829% (+0.04%) from 92.793%.

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[coveralls]

coverage: 92.829% (+0.04%) from 92.793% — refactor/sysv-arg-sequence into master