The Zig→Rust port is complete; the .zig files were kept as a non-compiled
reference during the port. Drop them now that the port has merged.

A handful of .zig files are kept because the codegen pipeline still
references them at build time:
- src/runtime/bake/DevServer.zig — read by bake-codegen.ts (convertZigEnum)
- the 7 .zig siblings of *.bind.ts files — bindgen.ts requires the sibling
  to exist
- the .zig files named in $zig()/$newZigFunction() callsites in src/js/ —
  generate-js2native.ts resolves them by path and throws if absent

Also:
- Regenerate test/internal/ban-limits.json for the smaller .zig file set
- Point test/js/bun/util/zstd.test.ts at parser.rs instead of the deleted
  parser.zig for its large-file test fixture