refactor(studio): make the fileset start selection a discriminated union

[steramae-nvidia]

What

CreateFilesetStart's onContinue took (optionId, templateId?), which can't carry a payload per option without accumulating more optional arguments. Replaced with a StartSelection discriminated union so each start option's payload travels inside its own case, and NewDataDesignerJobRoute switches on it exhaustively.

Also changes the Continue footer: it now appears as soon as an option is picked and disables itself with a hint ("Pick a recipe to continue."), instead of materializing only once the choice was already complete. Previously the button simply popped into existence with no indication of what was missing.

Why

PR 1 of 3 in a stack that adds the "Describe with AI" start option. This one is purely the contract change — the ai tile stays enabled: false and no new UI ships. Isolating it means the footer UX change (which is visible on the template path today) gets reviewed on its own rather than buried in a 1000-line feature diff.

Stack

1. → this PR — contract refactor
2. steramae/dd-ai-generation-hook — generation/validation/repair hook
3. steramae/dd-ai-start-option — the panel, and flipping the tile on

Testing

• pnpm --filter nemo-studio-ui typecheck — clean
• pnpm --filter nemo-studio-ui test src/components/CreateFilesetStart/index.test.tsx — 7 passing

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Added clearer guidance when a template selection cannot yet continue.
  • Continue actions now consistently support both scratch and template-based filesets.
  • Template selections proceed with the chosen template, while scratch selections follow the scratch workflow.

• Bug Fixes

  • Improved Continue button behavior so it remains disabled until required selections are complete.
  • Kept the action footer visible after selecting an option, making next steps clearer.

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ℹ️ Recent review info

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Run ID: 4387b58d-da5f-45d2-8134-e536b74e55bf

📥 Commits

Reviewing files that changed from the base of the PR and between 0cf4a0f and 06f5604.

📒 Files selected for processing (4)

• web/packages/studio/src/components/CreateFilesetStart/index.test.tsx
• web/packages/studio/src/components/CreateFilesetStart/index.tsx
• web/packages/studio/src/components/CreateFilesetStart/types.ts
• web/packages/studio/src/routes/NewDataDesignerJobRoute/index.tsx

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📝 Walkthrough

Walkthrough

Changes

Fileset start selection flow

Layer / File(s)	Summary
Selection contract and component behavior web/packages/studio/src/components/CreateFilesetStart/types.ts, web/packages/studio/src/components/CreateFilesetStart/index.tsx, web/packages/studio/src/components/CreateFilesetStart/index.test.tsx	StartSelection defines scratch and template payloads. The component emits these payloads, shows template guidance when no recipe is selected, and keeps Continue disabled until selection requirements are met. Tests cover the updated states and payloads.
Route handling for structured selections web/packages/studio/src/routes/NewDataDesignerJobRoute/index.tsx	The route accepts StartSelection and navigates according to optionId. Template navigation reads templateId from the selection object.

Possibly related PRs

• NVIDIA-NeMo/nemo-platform#530: Directly updates the same component, types, route integration, and tests.
• NVIDIA-NeMo/nemo-platform#537: Refines the same selection flow and onContinue contract.
• NVIDIA-NeMo/nemo-platform#961: Uses the same structured selection contract across the component and route.

Suggested reviewers: marcusds, htolentino-nvidia, aray12

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: refactoring fileset start selection into a discriminated union.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch steramae/dd-start-selection

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[github-actions]

Suite	Lines Covered	Line Rate	Branch Rate
Unit Tests	29440/37448	78.6%	63.2%
Integration Tests	17382/36166	48.1%	20.6%