Serialize session controls before prompts

[lyc-aon]

What changed

• serialize model, thinking, and fast control round-trips
• hold prompt, steer, and follow-up commands until pending control revisions settle
• prevent a fast mobile model-pick then Send tap from being rejected as stale
• cover the exact in-flight model-to-prompt ordering regression

Verified

• pnpm check
• pnpm test (781 tests)
• pnpm test:e2e (8 browser cases)
• web production build