Fix demux R channel backpressure

[phsauter]

When rready is used, obi_demux could expose a downstream response directly to the upstream manager and then let it change while the manager was stalled (rready is not there but demux moves on).
Add a one-entry response hold stage so stalled R-channel payload remains stable until accepted.

[micprog]

Nice catch! I'm wondering if it would make more sense to stall the request that changes the select_q instead of buffering the full response inside the demux - the same performance could be achieved then by adding a cut module or a module based on a fall-through reg, enabling flexibility and tighter area overhead control.

[phsauter]

I felt like I was missing some better way to do it, yeah I think I agree, your proposal is better. I will change it accordingly.

[phsauter]

OUTDATED

Your proposal alone did not fix the bug, so I started again from the spec:
OBI UseRReady makes the R channel a valid/ready handshaked channel.
Once a response phase is active, rvalid and the response payload must remain stable while rvalid && !rready, and the response must only retire once the rvalid && rready transfer completes.
So we must keep routing state stable during a stalled R phase.

MUX:
The mux now tracks the active response owner. While a response is stalled, it keeps routing rvalid/r to that same subordinate port and derives upstream rready from that owner.
The owners FIFO is only popped on rvalid && rready, so the next response owner cannot be selected before the current response transfer completes.

DEMUX:
The demux now tracks the active response source when UseRReady is enabled.
While a response is stalled, it keeps the upstream response selected from that same manager port and blocks switching requests to a different manager source until the stalled response phase completes. (basically what you suggested)
The in-flight counter retires responses only on rvalid && rready.

This adds state per port on both mux and demux. Its not insignificant but I was not able to find a better solution that guarantees compliance without tanking performance.

[phsauter]

Turns out if you are not tired this is actually way easier to fix and doesn't cost more state at all.
Thanks on pushing back on my tried and lazy ass ;)