fix: prevent skipping incomplete loop steps

[MattPua]

Problem

Users can bypass required fields in the new loop flow by clicking later steps in the progress indicator.

Changes

• Block forward step navigation until intervening steps are complete
• Keep backward navigation available
• Add focused navigation tests
• No visual change

How did you test this?

• pnpm exec vitest run packages/ui/src/features/loops/components/LoopForm.test.ts
• pnpm exec biome check packages/ui/src/features/loops/components/LoopForm.tsx packages/ui/src/features/loops/components/LoopForm.test.ts packages/ui/src/features/loops/components/loopStepNavigation.ts
• pnpm --filter @posthog/ui typecheck
• Full pnpm typecheck via pre-commit hook

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 25f49bd.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "refactor: inline loop step navigation gu..." | Re-trigger Greptile}

[github-actions]

Trivial, contained UI fix to step navigation gating with added tests; no risky territory touched and no unresolved concerns.

• Author wrote 0% of the modified lines and has 6 merged PRs in these paths (familiarity MODERATE).
• 👍 on the PR from greptile-apps[bot].

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	11L, 1F substantive — within ceiling
tier	✓	T1-agent / T1a-trivial (11L, 1F, single-area, fix)
stamphog 2.0.0b3		.stamphog/policy.yml @ dec9229 · reviewed head 25f49bd