Bound Watch remote-load test gates

[cbusillo]

Summary

• replace five unbounded Watch remote-load start spins with a continuation-backed actor signal
• bound missing-start failures with a named timeout while preserving the separate negative coalescing yield assertion
• add a direct timeout regression test

Refs #578

Validation

• six focused Watch tests passed
• 25 repeated focused stability runs passed
• loader short-circuit mutation failed with the expected named start timeout instead of hanging
• routine Swift lane passed 893 tests
• scripts/commit-gate.sh
• JetBrains changed-files inspection found only pre-existing spelling hits on unchanged product identifiers; no semantic Swift finding
• Opus and Gemini exact-SHA final reviews approved 534583a

Scope

• test-only change in Tests/ContextPanelCoreTests/WatchCompanionCacheTests.swift
• no production Watch loader or cache behavior changed