[Bug]: Copied programs reject bus references from the schema they were built with

[fedonman]

Bug Description

_validate_bus compares a BusRef's schema against the program's with is, but deepcopy, loads and with_waveforms all hand back a program holding a fresh copy of the BusSchema. Every ref built from the original schema is then rejected as foreign, so you cannot keep building on a program derived from another one. BusSchema defines no __eq__, so identity is currently the only comparison available; giving it structural equality and comparing with == would fix all three paths.

Minimal Reproducible Example

import copy
import qprogram as qp
from qprogram.buses import BusSchema

s = BusSchema.transmon()
p = qp.QProgram(schema=s)
p.measure(s.q[0].readout, "r", "w")
copy.deepcopy(p).measure(s.q[0].readout, "r", "w")

Expected Behavior

A copied, reloaded or waveform-bound program keeps accepting bus references from the same schema it was built with.

Actual Behavior

File "src/qprogram/qprogram.py", line 925, in _validate_bus
    raise ValidationError(msg)
qprogram.errors.ValidationError: BusRef 'q0/readout' (element='q', kind='readout') comes from a different BusSchema than the one attached to this QProgram. A program may use only one schema; use a plain string bus name if you need to reference a bus that lives outside the schema.

System Information

qprogram 0.1.0
python   3.14.3 | Linux-7.0.0-30-generic-x86_64-with-glibc2.43
vendors  none