Self-host link: remaining LLVM verify after stub progress (#816)

[PurHur]

Context

make bootstrap-selfhost-probe on fix/selfhost-native-link-milestone advances past Native compileBoolTemporary missing-arg failures but still fails native -o build/selfhost with:

Module verification failed due to Function return type does not match operand type of return inst!

Done in PR

• JIT stub return alignment (emitSelfHostStubReturn, cfgFunctionReturnCallbackType)
• Expanded Compiler coalesce/dim-fetch/Web/Runtime skip patterns
• Native::missingCallArg() for stub call sites

Remaining

• LLVM verify on functions still fully compiled in compiler_minimal bundle (likely Block, Func, or echo path dominate/return mismatches)
• ./script/bootstrap-selfhost-link.sh → runnable build/selfhost printing compiler_minimal bundle OK
• README milestone row update only after link script works end-to-end

Repro

make bootstrap-selfhost-probe
# NEXT_LOWER: Module verification failed...