Honor downstream demand in coalesceLatest (fixes launch trap)

[ejc3]

cmux traps on launch as soon as the sidebar starts observing a workspace:

Combine/Publisher+AsyncSequence.swift:112: Fatal error: Received an output without requesting demand

Three launches out of three here, dead within five seconds. The cmux-unit test
host dies the same way before a single test runs, so the suite can't be run
either.

Root cause

sidebarImmediateObservationPublisher ends in coalesceLatest, and the sidebar
reads it through .values. An AsyncPublisher requests one value at a time and
traps on any value it did not request, while coalesceLatest forwards every
value the moment it arrives.

receive(subscription:) hands the subscription downstream and then requests
unlimited demand upstream. @Published replays current state synchronously
inside that request, when downstream demand is still zero, so the replay traps on
arrival. The operator has forwarded without demand since #6807 added it; the
sidebar moved onto .values in #8211, which is when it started mattering.

Fix

Track downstream demand and forward only against it. A value with no demand
behind it becomes the coalesced pending value and is forwarded once demand
arrives, which is already what the operator does with a value that lands inside
an open window, and it holds at most one value either way. Forwarding on demand
does not re-stamp the window: the interval paces upstream bursts, not the
consumer's pull rate.

sink requests unlimited demand, so every existing subscriber behaves exactly as
before, and the sixteen tests that were already here pass untouched.

Why nothing caught it

The precondition was a comment — "sink-style subscribers only", "downstream
demand is ignored" — and nothing enforced it. Every existing test subscribed with
sink, whose unlimited demand is the one subscriber shape that cannot observe
the violation, so the operator's own tests were green by construction. Moving the
sidebar onto .values broke launch rather than failing a test. Honoring demand
puts the contract in the code, where a future subscriber can't quietly void it.

Tests

The failing test lands first, then the fix. coalesceLatestDeliversNoMoreValuesThanDownstreamDemanded
subscribes with an explicit demand of one value and records what arrives; against
the operator as it stands today:

✘ Expectation failed: (subscriber.received → [1, 2]) == [1]

coalesceLatestForwardsValueHeldForDemandOnceDemandArrives pins the other half,
so a fix that simply drops the undemanded value doesn't pass either.

With the fix, WorkspaceSidebarObservationTests is 18/18 and the app launches
and stays up.

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Summary by cubic

Fixes a launch crash by honoring downstream demand in coalesceLatest. This prevents “Received an output without requesting demand” when the sidebar consumes the stream via .values.

• Bug Fixes
  • Track downstream demand in coalesceLatest; forward only when there is demand. If none, hold the latest and deliver once demand arrives without re-stamping the window.
  • sink behavior is unchanged (unlimited demand).
  • Added demand-limited tests to cover this path; sidebar tests pass and the app now launches reliably.

^{Written for commit 16e2af5. Summary will update on new commits.}

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