std: drop the redundant flat mem.sx import from the facade

The flat #import of mem.sx predated the namespace tail — the tail's
mem :: #import already puts mem.sx in the program graph, which is all
the ufcs helpers (context.allocator.create/alloc/free/clone) and the
CAllocator default-context machinery need; std.sx itself references no
mem name. Probe-verified the full mem surface + all gates: suite
588/588, zig build test 0, m3te 23/23, game builds + bundles. The
double import was also duplicating lowered IR — the 37 re-pinned .ir
snapshots net ~2.5k lines smaller; output streams byte-identical.

library/modules/std.sx

@@ -11,8 +11,6 @@ core :: #import "modules/std/core.sx";
 fmt  :: #import "modules/std/fmt.sx";
 list :: #import "modules/std/list.sx";
 
-#import "modules/std/mem.sx";
-
 // --- core: builtins, libc escape hatch, compiler-resolved types ---
 // (`string` has no alias here: it is a reserved type name — its #builtin
 // declaration in core.sx resolves program-wide and cannot be re-bound.)