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/0713-modules-json-writer.sx

@@ -18,9 +18,9 @@
 // and freed in one `deinit`; the writer path allocates nothing.
 
 #import "modules/std.sx";
-#import "modules/allocators.sx"; // `Allocator` is non-transitive: name it, import it.
+#import "modules/std/mem.sx"; // `Allocator` is non-transitive: name it, import it.
 #import "modules/std/json.sx";
-#import "modules/fs.sx";
+#import "modules/std/fs.sx";
 
 // The exact document the writer must produce (insertion order, escaping).
 EXPECT :: "{\"name\":\"a\\\"b\\n\",\"tab\":\"x\\ty\",\"bs\":\"c\\\\d\",\"ctrl\":\"\\u0001\",\"n\":-7,\"zero\":0,\"pos\":7,\"min\":-9223372036854775808,\"max\":9223372036854775807,\"ok\":true,\"nil\":null,\"xs\":[1,-2,3],\"nested\":{\"k\":\"v\"}}";