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.

test/wcc/696_modtype_leaf_collision.c

@@ -15,8 +15,8 @@
  *
  * Fixtures live in test/wcc/data/modcollision/. mod1/mod1.ww and
  * mod2/mod2.ww are deliberately *new* source files so the test
- * doesn't lean on bufio's `bstream` workaround (still in place
- * until #21 lands).
+ * stays self-contained — independent of lib/bufio (now that
+ * `bufio.stream` and `io.stream` coexist on the post-#15 surface).
  */
 #include <stdio.h>
 #include <stdlib.h>