They're both declared in the same module - you can see them at
INFO: [Synth 8-5545] ROM "stack_reg[0]" won't be mapped to RAM because address size (32) is larger than maximum supported(25)
Both 'zp' and 'stack' are in fact written to - you can see the logic in `EXECUTE in 6502.v above under `STORE_MEM. I'm not sure why it thinks it's a ROM. Unless that logic is being optimised away for the time being, of course. I haven't checked that yet.
Cheers Simon