Fix: stagger concurrent-dispatch emits to eliminate Windows multiplex race

Summary

• Round-10 fix for Listening_ConcurrentDispatches_OutputsIsolatedByTicket — stagger emit timestamps (sleep 2/4/6s instead of uniform 2s) so all 3 markers hit the agent's output pipeline at distinct moments
• Prior round-9 (uniform SleepThenEcho-2s) was still flaking — Phase 13 PR-3 cleanup re-run (workflow run 25543530379) saw resultB.AllText empty even though ExitCode=0, meaning dispatch B's script ran cleanly but its log lines never surfaced through Halibut
• Concurrent-isolation contract still exercised — all 3 scripts run simultaneously during their sleep phases

Diagnosis

With all 3 emits at ~T+2.3s (within ~200ms of each other), the agent's LocalScriptService was multiplexing 3 simultaneous stdout bursts. If anything in its per-ticket queue / observer routing races, the middle ticket is the most likely to lose output. Empirical pattern: resultB.AllText == "" with resultB.ExitCode == 0.

Round 10 timing

Time	Activity
T=0ms	Dispatch A starts (sleep 2)
T=100ms	Dispatch B starts (sleep 4)
T=200ms	Dispatch C starts (sleep 6)
T≈2.3s	A emits "ticket-A-marker" — agent's pipeline handles ALONE
T≈4.4s	B emits "ticket-B-marker" — agent's pipeline handles ALONE
T≈6.5s	C emits "ticket-C-marker" — agent's pipeline handles ALONE

Concurrent overlap window: T=0.2s to T=2.3s (all 3 running, all 3 sleeping = 2.1s of genuine concurrent execution). Then A finishes; B+C still running. Then B finishes; C still running. Concurrent isolation contract still exercised.

Trade-off

Total wall-clock ~7s (was ~3s). Acceptable — flakes burn more CI time than this slow-down does (PR re-runs, retries, debugging).

Test plan

• CI on windows-latest runs Category=TentacleDeployE2E:
  • Listening_ConcurrentDispatches_OutputsIsolatedByTicket passes consistently across multiple runs
• dotnet build tests/Squid.WindowsTentacleE2ETests/Squid.WindowsTentacleE2ETests.csproj — 0 errors

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