checker: `-no-parallel` with a `-prealloc` compiler skips whole-program passes (deprecated `byte` not reported, goto label errors degrade to C errors)

Describe the bug

With -no-parallel, a -prealloc compiler (what v self builds) runs TypeChecker.check_semantics_scoped_serial instead of check_semantics, and that path does not run several whole-program passes of check_semantics. Two visible consequences:

• byte(1) compiles without the byte is deprecated, use u8 instead error.
• goto nowhere is no longer reported by the checker (unknown label); the C compiler reports it instead (label '__v_user_goto_0' used but not defined).

check_semantics (vlib/v/types/checker.v) runs check_import_diagnostics, check_c_js_generic_declarations, check_duplicate_fn_declarations, check_deprecated_byte_types, check_interface_reserved_parameter_names, check_goto_labels and check_labelled_loop_controls. check_semantics_scoped_serial (vlib/v/types/checker_parallel.v) runs check_export_attrs, check_top_level_declarations and the scoped batches; none of the others are referenced from checker_parallel.v. Some are evidently covered elsewhere (a duplicate fn is still reported), check_deprecated_byte_types and check_goto_labels are not.

The selection happens in the !want_parallel branch: if tc.scope_parallel_check_workers { tc.check_semantics_scoped_serial() } else { tc.check_semantics() }, and scope_parallel_check_workers is set by enable_scoped_parallel_workers() when the driver's should_scope_prealloc_stages() ($if prealloc) is true.

Because the diagnostic fixture harness (vlib/v/fixturetest) always passes -no-parallel, the three vlib/v/checker/tests/use_byte_instead_of_u8_*_err.vv fixtures produce no error at all under a -prealloc compiler.

Reproduction Steps

fn main() {
	x := byte(1)
	println(x)
}

v -o out byte_type.v                # error: byte is deprecated, use u8 instead
v -no-parallel -o out byte_type.v   # compiles, exit 0

fn main() {
	unsafe {
		goto nowhere
	}
}

v -o out goto_missing.v                # goto_missing.v:3:8: error: unknown label `nowhere`
v -no-parallel -o out goto_missing.v   # C compiler: label '__v_user_goto_0' used but not defined

Expected Behavior

-no-parallel only changes scheduling; the same diagnostics are produced as in the default mode.

Current Behavior

See above: the deprecated-byte error is dropped entirely, the goto-label error degrades to a C compiler error.

Possible Solution

Run the whole-program passes that check_semantics runs (or the subset not covered elsewhere — at least check_deprecated_byte_types and check_goto_labels) from check_semantics_scoped_serial as well, or factor them into a helper both entry points call. check_semantics_parallel should be audited for the same gap.

Additional Information/Context

Found while adding a byte-in-asm fixture for #28764: the fixture (and the existing use_byte_instead_of_u8_*_err.vv ones) cannot be validated by the harness on a -prealloc compiler.

V version

V 0.5.2 01a8af9 (master, 2026-09-18)

Environment details

Windows 11 amd64, tcc; v.exe built by v self (prealloc).

[!NOTE]
You can use the 👍 reaction to increase the issue's priority for developers.

Please note that only the 👍 reaction to the issue itself counts as a vote.
Other reactions and those to comments will not be taken into account.