lib+test: bufio rename bstream → stream

Validates the #15 same-leaf-name cross-module type fix (9d85aa4):
`bufio.stream` and `io.stream` now coexist on a `use bufio; use io;`
surface — bufiotest.ww references both in the same scope (e.g.
`let m: io.stream; let b: bufio.stream;`) and compiles clean.

Lifts the bufio-side workaround that bstream existed to dodge.
@test fns rename in lockstep (bstreamsmallwrite → streamsmallwrite,
etc.). Also tidies the stale "until #21 lands" parenthetical in
test/wcc/696_modtype_leaf_collision.c, since #21 is this commit.

make test: 54/54; bootstrap fixed-point 990–997 holds.

lib/log/log.ww

@@ -107,7 +107,7 @@ export type logger = struct {
 
 // stdlogger — concrete logger forwarding to a `*io.stream` sink.
 // First-field embed: `&sl.logger` yields a `*logger` (same shape as
-// bufio.bstream over its embedded io.stream vtable). The vtable
+// bufio.stream over its embedded io.stream vtable). The vtable
 // callback recovers the outer stdlogger by casting the dispatch arg
 // back to `*stdlogger`.
 export type stdlogger = struct {