cgen + memio: cgoutarena → memio.dynamic, grow → dynamicgrow (β-3)

Phase 0 last β-shape site. Two concerns in one commit because the
refactor surfaced the rename:

 - selfhost/cmd/wcc/cgen.ww  cgout buffer (cgoutbuf/cap/len + arena +
   cgout_grow + CGOUT_INIT_CAP) → memio.state + io.stream behind a
   one-shot lazy-init guard. cgout_enable drops its *arena param;
   memio.reset in cgout_flush keeps the buffer sticky across fns so
   the arena's amortisation survives — re-init per fn would abandon
   the buffer and re-grow from 0 via the 8→…→65536 ladder for every
   function (no io.close path → no os.free).

 - lib/memio/memio.ww  private fn grow → dynamicgrow. Symmetric with
   dynamicwrite / dynamicclose; required because cstage bundles all
   imported modules into a flat TU and resolves private fns by
   unqualified name, so the new `import memio;` in wcc's bundle
   collided with selfhost/cmd/wcc/mem.ww's arena `grow`. Module-aware
   private-fn scoping in cstage is task #9.

@test fn dynamicgrow in memiotest.ww (same package as memio.ww)
renamed to dynamicgrowcases to free the name; new suffix mirrors the
file's existing fixedwritecases / borrowedreadcases convention.

Lazy-init guard cgoutinit. memio.dynamic runs once on first
cgout_enable; subsequent enables just set cgoutmode. Mirrors
lib/log/log.ww:124 ensureinit. Without it, ~14 mmap syscalls per fn
and ~100 MiB+ cumulative leak on a typical bootstrap.

io.write bare discard in emitbytes mirrors lib/log/log.ww:169 —
memio.dynamicwrite never returns io.closed (memio.ww:166).

Verified 132/132 incl. 995_self_rebuild byte-identity.

selfhost/cmd/wcc/cgendecl.ww

@@ -3,7 +3,7 @@
 // Houses the top-level emission glue:
 //   - cgfnparams: parameter spilling per SysV
 //   - cgfn:       fn body emit (TEXT/SUBQ patched after body), prologue
-//                 deferred via cgen.ww's cgoutbuf so the frame size
+//                 deferred via cgen.ww's cgoutstate so the frame size
 //                 reflects every emit-time localadd (#15/#26c)
 //   - cgfile:     file-level entry (the exported driver)
 //
@@ -350,7 +350,7 @@ fn cgfn(c: *cgen, fn_: *node) void = {
 	// through *(@sretarg) and returns @sretarg in RAX.
 	let sret_callee: bool = sretretsize(c, c.fnret) > 0;
 
-	// Capture the body into cgoutbuf while c.frame grows under
+	// Capture the body into cgoutstate while c.frame grows under
 	// emit-time localadd calls (#15/#26c — wwstage dropped its
 	// scanlocals pre-pass to align DOWN with cstage's first-use
 	// pattern). The prologue (TEXT label, PUSHQ/MOVQ/SUBQ) emits
@@ -358,7 +358,7 @@ fn cgfn(c: *cgen, fn_: *node) void = {
 	// localadd. Mirrors cstage cmd/w6c/cgen.c cgfn which builds
 	// `subsp`/`text` Progs up front and patches their `from.offset`
 	// at the end via txt_emit.
-	cgout_enable(c.a);
+	cgout_enable();
 
 	if (sret_callee) {
 		let saoff: i32 = localadd(c, "@sretarg", 8, nil);