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.
Hojun-Cho
b842f9337b