Phase-2 (950/990-997) was blanket-serial only because ww_ww writes build intermediates next to source (#15), so gates race on selfhost/cmd/<tool>/main.* stems. Per-gate FS-footprint audit (ken): the reader group {990,991,992,994,996,997} writes only /tmp or disjoint tracked stems (smoke/mandelbrot) — parallelise race-free via the Phase-1 xargs -P machinery; the source-tree writers {993,995,950} stay a serial tail; combined_ww_fresh last. Verdict set identical (byte-id-neutral — parallelism alters scheduling, not emitted bytes); 2 stable green runs (237 each). make test ~459s -> ~390s.