build_one/buildone gain an explicit objstem: 'ww build -o X' derives
main.{combined.ww,s,o} beside X; 'ww run' uses its per-pid /tmp stem;
default no--o stays next-to-source (load-bearing for the make regen of
tracked combined.ww and the freshness gate). Realizes the redirect TODO
in 995_self_rebuild.c. Kills the concurrent-test torn-read race on
selfhost/cmd/<tool>/main.* (the 991 transient). 993/995/949 pass -o
into their per-pid workdirs; 949's '-o /dev/null' relied on the old
driver ignoring -o (audited: the only live driver case). Atomic
combined write (os.rename) deferred to its own commit - lib/os lacks
rename and that addition is an import-floor regen.
Both driver twins: an import that neither locates as a file nor is
satisfied by an inline 'package <name>' declaration in the unit is now
fatal "ww: cannot find package <name>" (was a silent skip that masked
dead imports and typos). The inline scan is a new every-line helper on
the uncapped comment-skip core - peekpackage stops at the first decl,
and single-file multi-package fixtures declare several. 949 pins both
branches (miss->fatal, inline->build+run); 993 adds ww_ww parity.
