ffi M5.A.next.4B.A: compile_error intrinsic — expected-failing lock-in

`compile_error(msg)` raises a build-time diagnostic at the call site
with `msg` as the error text. The arg must be a string literal —
runtime expressions can't be reported as compile errors. Used by
builder fns to reject malformed pack shapes / arg combinations
cleanly instead of silently emitting wrong code.

Today: `unresolved 'compile_error'`. Expected (post-fix): focused
diagnostic with the literal message at the call site's span. The
next commit adds the lowering arm.

tests/expected/187-compile-error.exit

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+1

tests/expected/187-compile-error.txt

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+/Users/agra/projects/sx/examples/187-compile-error.sx:12:6: error: intentional compile error from #run