fix(acp): re-subscribe channels the relay layer has dropped - #5014

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## Summary

When a Buzz agent falls behind on incoming messages, its connection can
make the backlog worse while trying to recover. The connection buffers
messages from the relay server until the agent is ready to process them;
if that buffer overflows, recovery previously requested history for
**every subscribed channel** and paused socket reads while sending those
requests. That adds traffic to an already overloaded connection. This
change requests history only for affected subscriptions, once the code
consuming those messages has room, with at least five seconds between
attempts.

The recovery path now:

- Combines repeated losses into one pending recovery per affected
subscription, keeping the oldest dropped timestamp so replay starts
early enough.
- Waits until at least half the consumer queue is free and the relay's
existing rate-limit delay has expired. The queue wakes recovery when
space becomes available; recovery does not periodically sample capacity
or hold queue space away from live messages.
- Attempts one subscription at a time, choosing the least recently
attempted so a busy channel cannot crowd out other channels or
membership notifications. The five-second delay starts when an attempt
finishes, including a failed write; failed writes leave recovery
pending.

Recovery is paced by available capacity, not by how often messages are
lost. This is not a larger buffer or a cutoff that abandons recovery.
Subscription identifiers, message filters, replay timestamp overlap and
duplicate filtering are unchanged; no downstream agent changes are
required.

This targets a reproducible overload **amplifier**, not every cause of
overload or every catch-up limitation. The initial live overload's cause
has not been established. Recovery remains best effort: a successful
request write is not proof of delivery, and existing history/retention
limits, bounded duplicate tracking and replay limitations still apply.
There is no exactly-once or complete catch-up guarantee. A stalled write
can still pause socket reads for the existing ten-second timeout; the
pacing bound does not cover initial subscriptions, reconnects or other
retry paths.

### Related issue

Closest related: #5014 (channel re-subscription); also #6661 (membership
reconciliation) and #6090 (relay backpressure gap signaling). This
addresses local overflow recovery scheduling, not those separate
mechanisms.

### Testing

Recorded offline comparisons against the previous behavior, with the
final implementation at `8000636f3073167c5a5107bb179c7d91160f1729`:

| Same fixture: 18 subscriptions, three overload rounds | Before | After
|
| --- | --- | --- |
| Recovery history requests | 108 | 3 |
| Ping-response delay | About 4.6 seconds | Below the measurement's 1 ms
resolution |

A separate bounded-history fixture delivered all 320 events plus
subsequent live traffic in **both** versions. Regression coverage
exercises the real socket-handling task, including intermittent consumer
capacity, fairness, failed writes and cancellation of capacity waits
before live delivery. These are synthetic results, not production
throughput measurements or evidence of a deployed cure.

The full local `RUST_TEST_THREADS=4 just ci` run passed on September 4,
2026. Earlier unsuccessful local runs remain part of the validation
history. The [recorded validation evidence and separate desktop
follow-up](#7325 (comment))
preserve the original desktop mock-history scroll failure, its passing
rerun and the remaining investigation. That desktop path does not run
the agent connection code; neither this repair nor the passing rerun
fixes the observed scroll problem.

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Signed-off-by: Logan Johnson <loganj@squareup.com>