DualPi2: Implement WRR credit scheduler

Implements the RFC 9332 / Linux sch_dualpi2 byte-weighted round-robin
(WRR) credit scheduler (Config::c_protection, defaulting to 10% weight
for the classic queue and 90% weight for the L4S queue):

1. Prevents classic queue starvation: Previously, l4s_queue_ had strict
   priority over classic_queue_, allowing sustained L4S traffic to
   starve non-ECT packets (such as ICE STUN pings) in classic_queue_
   and inflate RTT measurements.
2. Coupled queue delay sampling: Updates UpdateBaseMarkingProbability()
   to sample max(l4s_queue_delay, classic_queue_delay) per RFC 9332 so
   alternating dequeues between classic and L4S queues do not cause
   negative integral updates while both queues are backlogged.

Bug: webrtc:447037083, webrtc:42225697
Change-Id: Ie0cc4fb39ad9e5c9f412a2046f648381f94d5f13
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/508180
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48842}