fix(hub): surface fanout slow-listener disconnects

[CoderMungan]

Closes #94.

Covers items #1 (make the drop-counter observable) and #2 (regression
test) from the issue. Item #3 (configurable fanOutBuffer) is deliberately
left out — as the issue puts it, tuning the constant before the counter is
observable would be tuning blind.

#1 — f.dropped is no longer dead

The counter is now readable on two surfaces, both options the issue listed:

Warning at the moment of disconnect. broadcast() calls logWarn.Warn
when it cuts a slow listener loose, using a new HubFanOutSlowListener
format constant in internal/config/warn (same pattern as the existing
HubReplicate* family — no literal in the hub package). The message carries
the cumulative count so log aggregators can rate it:

ctx: hub fanout: disconnected slow listener (buffer full); cumulative disconnects: 3

Cumulative count on the Status RPC. StatusResponse gains
DroppedListeners uint64 (json:"dropped_listeners"), populated by
hubStatus from a new droppedCount() accessor. ctx hub status prints

Dropped listeners: 3 (slow subscribers disconnected)

only when the count is non-zero, so a healthy hub's output is byte-for-byte
what it was before and no existing test or doc example changes.

On sync/atomic

Per the third acceptance item, broadcast increments with
atomic.AddUint64 (its return value is what the warning reports) and
droppedCount reads with atomic.LoadUint64, so the Status RPC handler
reads the counter without contending with an in-flight broadcast.

Every release target in hack/build-all.sh is 64-bit (darwin, linux and
windows on amd64/arm64), so the struct-field alignment caveat that applies to
the raw atomic.*Uint64 helpers on 32-bit platforms doesn't bite here.

#2 — regression test

TestFanOut_DisconnectsSlowListener overflows the buffer against a listener
that never drains, then asserts all three halves of the contract: the channel
is closed, the subscriber is gone from f.subs, and the counter incremented.
TestFanOut_DroppedCountStartsAtZero pins the healthy path so the counter
can't start drifting upward.

TestFanOut_DroppedCountRaceWithBroadcast covers the concurrency the
acceptance item is really about: four goroutines call droppedCount() — the
Status RPC handler's read path — while broadcast disconnects listeners.

I verified both by mutation rather than trusting them:

• deleting the delete(f.subs, ch); close(ch) block fails
  TestFanOut_DisconnectsSlowListener (count = 1, want 0 after disconnect
  and disconnected channel never closed) while the original three tests
  still pass — the exact silent regression the issue describes;
• reverting the counter to a plain f.dropped++ / return f.dropped makes
  go test -race report WARNING: DATA RACE and fail
  TestFanOut_DroppedCountRaceWithBroadcast.

The tests redirect logWarn.SetSink(io.Discard) so the new warning doesn't
pollute test output.

The rendered line is pinned too

desc.Text returns "" for an unknown key, so a renamed text key would blank
the new ctx hub status line silently — the same shape of bug this issue is
about. internal/write/hub gains TestClusterStatus_DroppedListeners
(asserts the rendered count) and TestClusterStatus_NoDroppedListeners
(asserts the omission at zero, and that the existing stats line survives).
Renaming DescKeyWriteHubDroppedListeners fails the first one.

Docs

• docs/cli/hub.md — notes the conditional Dropped listeners: line.
• docs/operations/hub-failure-modes.md — new Slow Listener Disconnected
  entry under Network, explaining that the disconnect is the loss-prevention
  mechanism (the client reconnects with its last-seen sequence and the hub
  replays), what the warning and counter mean, and when a climbing count is
  worth acting on.
• internal/hub/doc.go — the Concurrency section said "slow subscribers are
  dropped", which read as entry loss; corrected to describe the disconnect
  and point at the counter.

Verification

Ran the CI commands verbatim:

Check	Result
CGO_ENABLED=0 go build ./...	clean
CGO_ENABLED=0 go test ./...	181 packages, 0 failures
go test -race ./internal/hub/	clean
CGO_ENABLED=0 go vet ./...	clean
golangci-lint run	0 issues
hack/lint-docstrings.sh	clean

Nothing here touches the shell, PowerShell, OpenCode-plugin, or VS Code
extension surfaces.