fix: mer init fails — process.spawn doesn't search PATH in Zig 0.16

[justrach]

mer init myapp crashes with FileNotFound when calling zig build because std.process.spawn in Zig 0.16 no longer searches PATH by default.

Reproduction:

mer init myapp
# error: FileNotFound at process.spawn

Root cause: In 0.15, Child.init + .spawn() searched PATH. In 0.16, process.spawn requires the full path or explicit PATH resolution via expand_arg0: .expand.

Fix: Add .expand_arg0 = .expand to SpawnOptions, or resolve zig path before spawning.