feat: SysV multi-word args and sret record returns

[rieske]

Summary

• Multi-word values (valueWords != 1) always travel on the stack; pad on word parity; callee homes advance by word count.
• Non-variadic records larger than 16 bytes use SysV sret: hidden pointer in rdi, copy to [__sret], rax holds that pointer. Returns of 8/16 bytes use rax[+rdx].
• Same productEmitsMemoryReturn predicate on caller and callee. Call IR carries memoryReturnDest / Retrieve memoryReturn.
• Call/return emit lives in StackMachine_Call.cpp. loadWord is the single word-addressing helper (sret excludes the dest pointer).
• StructsReturnTest covers Point/pair/strbuf-shaped/nested sret, register args after hidden pointer, and large pass+sret (5 6 7).

Test plan

• Local codegenTest, symbolsTest, typesTest green
• Local struct-return functional suite intel + att smoke
• CI build workflow (build + serial tests + coverage / Coveralls)

Coveralls: 92.885% (+0.06%) from 92.829%.

[mergify]

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