fix: Zig 0.16.0 compatibility

[zclawz]

Fixes Zig 0.16.0 build failure.

Problem

std.Build.Cache.Directory.cwd().handle.realpathAlloc no longer exists in Zig 0.16.0 — realpathAlloc was removed from Io.Dir.

Fix

Replace with b.allocator.dupe(u8, proto_path). This is safe because proto_path is obtained from LazyPath.getPath3(b, step).toString(b.allocator), which already returns an absolute path — the realpathAlloc call was only resolving symlinks/normalising, which is not needed in the build step context.

Also bumps minimum_zig_version to 0.16.0.