gitignore: lib/**/*.combined.ww; drop stray lex.combined.ww
`ww build` on a lib/ module drops a .combined.ww snapshot next to the source. Only the bootstrap-frozen copies under selfhost/cmd/*/main.combined.ww are intentionally tracked; the lib/ ones are transient. lex.combined.ww slipped in via an ad-hoc `git add lib/`.
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