Feature Request: Progressive ramp workflow with validation checkpoints

[jakesciotto]

Is your feature request related to a problem?

Teams want to start an experiment at a small rollout (for example 5%), confirm that exposure works and that no guardrail metric moves, and then increase the rollout in steps. Today this is a manual loop: change the rollout percentage on the flag, wait, open the exposures tab, check the split and the $multiple count, check the guardrail metrics, and repeat. Nothing in the product guides or records this loop. Existing work covers parts of it: SRM explanation (PR #106289), $multiple warning (#59710), guardrail alerts (#67202), and alerts that can stop an experiment (#101995). No issue covers the ramp itself.

Describe the solution you'd like

1. A Ramp plan on the experiment: an ordered list of rollout steps, for example 5% → 25% → 50% → 100%, each with a minimum hold time.
2. A checkpoint between steps. PostHog shows a checklist for the current step: exposure split matches the rollout (SRM check), $multiple rate is under the threshold, exposures are counted in the expected event, no guardrail metric has a significant negative movement.
3. The user confirms each step by hand. Optionally, the step can advance automatically when all checks pass (opt-in).
4. Each step change is written to the activity log with the checkpoint results.
5. If a checkpoint fails, show the diagnosis and the next action (link to Experiment diagnostics: advise what to do #22101).

Describe alternatives you've considered

• Scheduled flag changes on the feature flag. They change the percentage, but do not validate anything.
• Alerts only (Auto-flag experiment metric changes (guardrail alerts + off-experiment anomalies) #67202, Act on experiment metric movement: alerts that can pause or stop an experiment #101995). They react to a problem, but do not structure the ramp.

Additional context

Related: #67202, #101995, #59710, #22101, #35750, PR #106289, PR #57767 (significance alerts).