Upgrade pg.zig pin to a Zig 0.16-compatible version (blocks nanoapi runtime adoption)

[justrach]

Problem

`zig/build.zig.zon` pins `pg.zig` at SHA `7605502f24e348c50341cbdf84ac135613d512af`:

```zig
.pg = .{
.url = "git+https://github.com/justrach/pg.zig?ref=master#7605502f24e348c50341cbdf84ac135613d512af\",
.hash = "pg-0.0.0-Wp_7gYTxBgDOvvLXaKfiXksk9WArPQJWgY833oQjfk1y",
},
```

That version of pg.zig calls `lib_test.addLibraryPath(p)` in its own build.zig — the `addLibraryPath` API on `Build.Step.Compile` was removed in Zig 0.16. The whole turboAPI build fails before even compiling the lib:

```
zig-pkg/pg-.../build.zig:73:21: error: no field or member function named 'addLibraryPath' in 'Build.Step.Compile'
lib_test.addLibraryPath(p);
```

Why now

This blocks adopting nanoapi's runtime layer as the shared HTTP base (see nanoapi#12). Wiring nanoapi into turbonet's main `build.zig` triggers full dep resolution including pg.zig, which fails. The `feat/nanoapi-runtime-spike` branch works around this by living in a self-contained `spike/` subproject — fine for proving the integration shape, not viable for the real port.

Resolution options

1. Bump `pg.zig` to a newer commit on `master` that fixes `addLibraryPath` for Zig 0.16. Check if upstream has already moved.
2. Fork `justrach/pg.zig` and patch the build.zig directly. Pin the fork.
3. Drop pg.zig from turbonet's build if TurboPG can be built as a separate Zig artifact — pg.zig only goes into the postgres path, not the HTTP runtime, so a build-time split might work cleanly.

Option 1 is preferred if a compatible upstream exists. Option 2 is the fast path. Option 3 is the right end state but more refactor.

Acceptance

• `python zig/build_turbonet.py --install` succeeds on Zig 0.16
• Adding `.nanoapi = .{ .path = "" }` to `zig/build.zig.zon` and importing nanoapi from `zig/src/main.zig` does not break the build
• TurboPG functionality (test suite under `tests/test_turbopg*.py`) still works

Blocks

turboAPI — the nanoapi runtime-layer port (PyDispatcher, removing the duplicated server.zig HTTP plumbing).

[justrach]

Fixed on feat/io-uring-linux — pg.zig is now vendored locally under zig/pg/ instead of being pulled as a remote dependency. The vendored version already has the Zig 0.16-compatible fix (lib_test.root_module.addLibraryPath instead of lib_test.addLibraryPath).

See commit e2e51cd: feat(turbopg): vendor justrach/pg.zig + add io_uring transport (opt-in, Linux).