lib/memio: add Option C parallel vstream API (#94 fold-e2)

Adds lib/memio/vstream.ww with three new constructors —
fixed_vstream / dynamic_vstream / dynamicfrom_vstream — that return
io.vstream (= *io.vtable, from lib/io/stream.ww) alongside the
pre-vtable memio.fixed / dynamic / dynamicfrom shape in memio.ww.

Hare's memio::fixed/dynamic return a `stream` whose first field IS
io::stream (= *vtable); ww mirrors that intrusively with fixed_ctx
+ dynamic_ctx structs whose first field is `vt: io.vtable`. A
heap-alloc'd *fixed_ctx is castable to vstream via `&c.vt`, and
callbacks recover the outer ctx via `s: *fixed_ctx` (same pattern
as lib/bufio.stream over io.stream and lib/log.stdlogger over
logger). ptr/len/cap kept flat (memio.ww:39 SOP) to dodge the
chained-dot-through-pointer-into-slice-subfield miscompile family.

Constructor flow: alloc with vt zero-initialised via a local, then
chained `c.vt.X = …` field-assigns through the *ctx pointer.
Struct-lit init via `vt = local_vt` (with local pre-set) silently
drops tagged-union slots past the first — sibling task filed,
workaround is the alloc-then-assign route (proven byte-id between
both stages). *ctx is a plain pointer-to-struct, not an aliased
pointer, so the chained-store path doesn't hit #195.

Cast workaround per #206 at each vtable-fn-ptr-slot init (8 sites
across the 3 constructors): bare `&fn_name` does not type-check as
`(*<alias> | void)`. Same `(&fn): *io.<role>` shape that
test/wcc/775_io_vtable_run.c uses. Drops out when #206 closes.

OLD memio surface is UNCHANGED. fold-eFinal (task #50) atomically
flips the package shape: deletes OLD constructors + callbacks and
renames `_vstream` suffix off.

Probe test/wcc/776_memio_vstream_run.c: 4 rows
(fixed_read_5 / dynamic_write_grow / dynamicfrom_alt_rw /
branched_fixed) exercise both vtable flavours through the
io.st_read / st_write / st_close dispatchers. Each row drives
cs runtime + ww runtime + cs.s == ww.s byte-id — 12 fixtures
total, all green.

Pre-existing wwstage gap surfaced + documented inline: ww_ww's
combined.ww concat order trips the wwstage checker on os.tryread
/ trywrite / tryopen's bare `return r;` over `(int | oserror)`
when os is checked after rt/io. Each probe row places `import os;`
FIRST to match the ordering selfhost uses (time → os → rt → …)
where the checker resolves cleanly. Sibling task; resolves the
ordering-sensitivity in the wwstage checker drops the workaround.

Makefile

@@ -113,7 +113,7 @@ $(BIN)/wwdump_ww: selfhost/cmd/wwdump/main.ww \
 		selfhost/cmd/wcc/cgen.ww selfhost/cmd/wcc/cgenexpr.ww \
 		selfhost/cmd/wcc/cgenstmt.ww selfhost/cmd/wcc/cgenutil.ww \
 		selfhost/cmd/wcc/cgendecl.ww \
-		lib/os/os.ww lib/rt/malloc.ww lib/time/time.ww lib/strconv/strconv.ww lib/strconv/stof_data.ww lib/strconv/decimal.ww lib/strconv/stof.ww lib/strconv/ftos_data.ww lib/strconv/ftos.ww lib/strings/strings.ww lib/bytes/bytes.ww lib/encoding/utf8/utf8.ww lib/ascii/ascii.ww lib/errors/errors.ww lib/io/io.ww lib/io/stream.ww lib/io/types.ww lib/memio/memio.ww lib/fmt/fmt.ww \
+		lib/os/os.ww lib/rt/malloc.ww lib/time/time.ww lib/strconv/strconv.ww lib/strconv/stof_data.ww lib/strconv/decimal.ww lib/strconv/stof.ww lib/strconv/ftos_data.ww lib/strconv/ftos.ww lib/strings/strings.ww lib/bytes/bytes.ww lib/encoding/utf8/utf8.ww lib/ascii/ascii.ww lib/errors/errors.ww lib/io/io.ww lib/io/stream.ww lib/io/types.ww lib/memio/memio.ww lib/memio/vstream.ww lib/fmt/fmt.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	@mkdir -p $(BIN)/wwdump_ww.d
@@ -135,7 +135,7 @@ $(BIN)/w6c_ww: selfhost/cmd/w6c/main.ww \
 		selfhost/cmd/wcc/cgen.ww selfhost/cmd/wcc/cgenexpr.ww \
 		selfhost/cmd/wcc/cgenstmt.ww selfhost/cmd/wcc/cgenutil.ww \
 		selfhost/cmd/wcc/cgendecl.ww \
-		lib/os/os.ww lib/rt/malloc.ww lib/time/time.ww lib/strconv/strconv.ww lib/strconv/stof_data.ww lib/strconv/decimal.ww lib/strconv/stof.ww lib/strconv/ftos_data.ww lib/strconv/ftos.ww lib/strings/strings.ww lib/bytes/bytes.ww lib/encoding/utf8/utf8.ww lib/ascii/ascii.ww lib/errors/errors.ww lib/io/io.ww lib/io/stream.ww lib/io/types.ww lib/memio/memio.ww lib/fmt/fmt.ww \
+		lib/os/os.ww lib/rt/malloc.ww lib/time/time.ww lib/strconv/strconv.ww lib/strconv/stof_data.ww lib/strconv/decimal.ww lib/strconv/stof.ww lib/strconv/ftos_data.ww lib/strconv/ftos.ww lib/strings/strings.ww lib/bytes/bytes.ww lib/encoding/utf8/utf8.ww lib/ascii/ascii.ww lib/errors/errors.ww lib/io/io.ww lib/io/stream.ww lib/io/types.ww lib/memio/memio.ww lib/memio/vstream.ww lib/fmt/fmt.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	@mkdir -p $(BIN)/w6c_ww.d
@@ -326,6 +326,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_isas_spread_variant \
 	  $(BIN)/test_tagged_widen_named_variant \
 	  $(BIN)/test_io_vtable_run \
+	  $(BIN)/test_memio_vstream_run \
 	  $(BIN)/test_use_promote_alias \
 	  $(BIN)/test_field_signed $(BIN)/test_frame_argcount \
 	  $(BIN)/test_selfhost $(BIN)/test_w6a_ww $(BIN)/test_w6l_ww \
@@ -688,6 +689,13 @@ $(BIN)/test_io_vtable_run: test/wcc/775_io_vtable_run.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_memio_vstream_run: test/wcc/776_memio_vstream_run.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		lib/memio/memio.ww lib/memio/vstream.ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_arrlit_str_full: test/wcc/711_arrlit_str_full.c \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \