std: restructure — std/ modules, namespace tail, std/xml.sx

allocators/fs/process/socket/log/trace/test move under modules/std/
(allocators.sx becomes std/mem.sx; the Allocator protocol moves into
the std.sx prelude, impls stay in mem.sx). New std/xml.sx holds
xml_escape as xml.escape. std.sx gains the carried namespace tail —
flat-importing std.sx now also provides mem./xml./log. — with the
remaining modules (fs/process/socket/json/cli/hash/test) deferred from
the tail until the global last-wins maps are fully own-wins (pulling
them into every closure collides bare names corpus-wide; they stay
direct imports: modules/std/fs.sx etc.). log.sx's internal emit
renamed log_emit (it clobbered consumer fns named emit program-wide).
bundle.sx uses xml.escape via the carried alias. Consumer import paths
swept mechanically; .ir snapshots recaptured for the larger std
closure. m3te + game build unchanged.

examples/0718-modules-cli-exit-json.sx

@@ -19,7 +19,7 @@
 
 #import "modules/std.sx";
 #import "modules/std/cli.sx";
-log :: #import "modules/log.sx";
+log :: #import "modules/std/log.sx";
 
 report :: (label: string, ok: bool) {
     if ok { print("{}: ok\n", label); } else { print("{}: FAIL\n", label); }