standby.ts: Add configurable warm-up period before standby mode activates

[Kingsman-99]

Description

src/standby.ts switches to standby mode immediately when inactivity is detected. During startup, normal initialization traffic can falsely trigger standby too early. A configurable warm-up period during which standby is suppressed prevents premature activation.

Acceptance Criteria

• StandbyController accepts a warmUpMs?: number option (default: 0, i.e., no warm-up)
• During the warm-up period, inactivity does not trigger standby mode
• After the warm-up period expires, standby detection resumes normally
• Unit tests pass

Context

• Target file: src/standby.ts
• Use src/timeMachine.ts to advance time in tests