The Foundation: Continuous Polling & Jitter Loop

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### Description:

Implement the ChangeDetector module and the foundational RolloutStateMachine. This slice establishes the core continuous polling mechanism using a recursive setTimeout loop that checks for configuration changes via If-None-Match (ETag) HTTP headers. To prevent cluster thundering herds, the polling interval must apply a +/- 15% randomized jitter. Update the BaseOptions API contract to accept pollIntervalMs, disableHotReload, and the onChange callback. When a change (200 OK) is detected, the SDK should deliver the payload directly to the onChange callback and update its internal ETag state.

### Acceptance criteria

- [ ] BaseOptions accepts the new parameters (pollIntervalMs, disableHotReload, onChange).

- [ ] Passing disableHotReload: true completely disables the polling loop.

- [ ] The polling mechanism uses recursive setTimeout with +/- 15% randomized jitter (verified via vi.useFakeTimers()).

- [ ] Detecting a 200 OK response successfully triggers the onChange callback with the new configuration.

### Blocked by:
None - can start immediately

User stories covered: 1, 2, 3, 10

Start by locating the ChangeDetector, RolloutStateMachine, and BaseOptions entry points, then trace the existing configuration request path. Use vi.useFakeTimers() to verify recursive polling, jitter, and the disabled-hot-reload case; done means the onChange callback receives a 200 OK payload and ETag state updates.

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