feat(runtime): engine-owned session cache; Flash-Next keeps many prompts warm (A/B/A 16.3 s → 4.9 s) - #825

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Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/