amalloc has 0 callers post-γ-2; the *arena threaded through ww
driver's importpathform / locatein / locateimport / enumeratedir /
peekpackage / arenadupcstr / builddirmodulepath / buildsearchpath /
resolvemodule and the expctx.a field are vestigial.
Drop `import mem;`, remove expctx.a, strip *arena from 9 signatures,
update 9 call sites. Drop 4 dead `let a: *arena = newarena();` in
buildone/dobuild/dorun/dotest. Comments retidied. main.combined.ww
auto-regenerated.
builddirmodulepath body intact — still 0 callers; γ-5b drops it.
arenadupcstr name is now a misnomer (task #10).
Verified 132/132 incl. 993_ww_ww + 995_self_rebuild byte-identity.