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/repo_transaction.sx

@@ -19,7 +19,7 @@
 //      are absent.
 // Uses a fresh `<root>` under `.sx-tmp/` and cleans up.
 #import "modules/std.sx";
-process :: #import "modules/process.sx";
+process :: #import "modules/std/process.sx";
 #import "../src/domain/platform.sx";
 #import "../src/domain/app.sx";
 #import "../src/domain/release.sx";