lib: split io empty stream into empty.ww per ref/hare/io/empty.ha

Pure file-boundary move of lib/io/stream.ww's single 'empty stream'
banner section (_empty_read, _empty_write, _empty_vt, empty) into
lib/io/empty.ww, mirroring ref/hare/io/empty.ha. No code, name,
signature, or behavior change. Comment-only adjustments: the banner
line is deleted (the file name carries it); the moved block's
'Lives in stream.ww' self-reference now says empty.ww; stream.ww's
head drops its empty bullet (content duplicated by empty.ww's own
comments); io.ww's ownership map gains the empty.ww line; types.ww's
'three files' count becomes four. empty.ww needs no imports (package
io types only).

Makefile: lib/io/empty.ww added beside every lib/io/stream.ww
occurrence (WWFIXTURE_SRC line 75; w6c_ww/wwdump_ww bootstrap prereq
lists at lines 184/204). lib/io has no _test.ww, so no LIBRARY_TESTS
or byteid-roster change; byteid coverage rides the bootstrap gates.

Validation: out/bin/w6c lib/io/io.ww (standalone contract) exit 0;
importer tests green: bufio 26, memio 11, log 11, getopt 12 (calls
io.empty), fmt 49.

lib/io/types.ww

@@ -6,7 +6,7 @@
 // the same: lib/io/io.ww owns the `eof` and `underread` tags;
 // lib/io/stream.ww owns the vtable surface (`vtable`, `stream`,
 // `read`/`write`/`close` dispatch); this file owns the surrounding
-// port. The three files share `package io;` so cross-file refs
+// port. The four files share `package io;` so cross-file refs
 // resolve via the dir-enum concat order (io.ww < stream.ww < types.ww
 // — `stream` lands before the reader/writer/closer aliases below).
 //