fix(cloud): let stats reads cancel through the shared read coordinator

[teamleaderleo]

Why

Three VMClientReadCoalescingTests cases fail on every main full-suite run since #13327 landed (06:35Z on 09-24):

• "Overlapping machine stats callers share one HTTP request"
• "A failed stats sample clears the last live reading"
• "An authoritative fleet replaces the old stats batch and preserves other readers"

The failures point at a real behavior regression, not just stale tests.

#13151 (merged ~03:00Z) added CloudReadRequestCoordinator. It already shares every concurrent /stats GET per account, team, and auth generation, and it cancels the HTTP read once its last caller leaves. #13327 was written before that landed. It added a second single-flight layer inside VMResourceStatsStore that runs the fetch in an unstructured Task. That layer has two effects:

• The coordinator now sees one waiter per machine instead of one per caller, so the tests' waiter accounting times out after 10 s.
• The store-level task never cancels. A removed machine or a hidden panel keeps its stats request alive until the 30 s timeout. The fleet test catches exactly that: /api/vm/removed/stats stays in flight after the fleet drops the machine.

What changed

This reverts #13327:

• VMClient.stats fences each caller with beginRead/finishRead. That keeps the revision and sequence ordering the store owns, so resize, reset, and removal still reject late completions.
• The network read is shared through the coordinator.
• VMResourceStatsStore.read(machineID:), its readTask field, and the tests that only covered that method are removed.

VMClientReadCoalescingTests covers the same ground at the client, with cancellation included: shared reads, hidden-panel cancellation, and resize invalidation.

Verification

Focused run of VMClientReadCoalescingTests and VMResourceStatsStoreTests on blacksmith-6vcpu-macos-15: 36 tests in 2 suites pass. run

🤖 Generated with Claude Code

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Summary by cubic

Fixes a stats read regression so machine stats requests cancel when their last caller leaves.

• Reverts the single-flight layer added in the previous stats store, which ran unstructured tasks that kept requests alive until the 30 s timeout after a machine was removed or a panel hidden.
• Stats reads now share the network request through the coordinator while fencing each caller with beginRead/finishRead to preserve revision and sequence ordering for resize, reset, and removal.
• Removes VMResourceStatsStore.read(machineID:), its readTask field, and the tests that only covered the store-level method; VMClientReadCoalescingTests covers sharing, cancellation, and resize invalidation at the client.

^{Written for commit 839a0c4. Summary will update on new commits.}

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📥 Commits

Reviewing files that changed from the base of the PR and between ea10f16 and 839a0c4.

📒 Files selected for processing (4)

• Sources/Cloud/VMClient+ResourceStats.swift
• Sources/Cloud/VMResourceStatsStore+Entry.swift
• Sources/Cloud/VMResourceStatsStore.swift
• cmuxTests/VMResourceStatsStoreTests.swift

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