sx sync: migrate legacy modules/{fs,process}.sx imports to modules/std/

The June stdlib restructure deleted the flat library modules; the old
paths kept resolving only through a stale zig-out/library install
snapshot. Verified green with that snapshot removed. Drop the empty
src/infra/ — the planned hash/json/cli shims shipped as sx std modules
instead.

tests/cli_dispatch.sx

@@ -21,7 +21,7 @@
 // =====================================================================
 
 #import "modules/std.sx";
-proc :: #import "modules/process.sx";
+proc :: #import "modules/std/process.sx";
 json :: #import "modules/std/json.sx";
 
 // True iff `needle` occurs in `hay`. Plain scan — the captured streams are