Core: Warn when multiple story files collapse onto one componentId

[valentinpalkovic]

Closes SB-1847

What I did

When several CSF files share one title, they all map to the same componentId (the title-derived prefix of every story id), and the component-entry selection keeps exactly one file per componentId - last one wins.
Every other file's component silently loses its docgen payload, story snippets, component manifest entry, and MCP docs, while the sidebar and preview render all stories normally, so nothing looks wrong.
This PR makes the data loss visible: the selection now warns once per collision, naming the colliding files and the winner.

button.stories.ts            (title: 'Example/Button', component: ButtonComponent)  ─┐
                                                                                     ├─> componentId 'example-button' ──> one entry survives ──> docgen / snippets /
button-secondary.stories.ts  (title: 'Example/Button', component: HeaderComponent) ──┘        (last set() wins)             manifest / MCP for ONE component only

The warning, captured from a real angular-vite/docgen-server-ts sandbox build with exactly that setup:

▲  Multiple story files share the component id 'example-button':
│  - ./src/stories/button-secondary.stories.ts
│  - ./src/stories/button.stories.ts
│  Component-level docs (props tables, code snippets, manifests, MCP
│  docs) for this id are generated from './src/stories/button.stories.ts'
│  only, so stories in the other files are left out of them. If these
│  files document different components, give each file a unique title so
│  every component keeps its docs.

The loss it points at, verified in the same build's artifacts: index.json lists 5 stories under example-button, but services/core/story-docs/example-button.json only carries the 4 stories from the winning file, and services/core/docgen/example-button.json documents ButtonComponent only - HeaderComponent's 4 documented inputs are gone.
After giving the second file a unique title, the warning disappears and both components come back with their full argTypes.

The single decision that matters is the collision predicate - distinct importPaths of eligible story entries per componentId - paired with once.warn for deduplication, so the several services that share this selection (docgen, story-docs, manifests, MCP access, refresh subscriptions) don't repeat the same warning:

for (const [componentId, importPaths] of storyImportPathsByComponentId) {
  const winner = entriesByComponentId.get(componentId);
  if (importPaths.size > 1 && winner) {
    once.warn(buildCollisionWarning(componentId, importPaths, winner));
  }
}

once deduplicates on the exact message, and the message contains the sorted file list plus the winner - so the same collision warns once per process regardless of index order, while a changed file set or a changed winner produces a fresh, corrected warning. Each of those properties is pinned by a unit test (a mutation probe confirmed the earlier hand-rolled dedup key was untestable, which is why this now reuses once instead).

Two things worth knowing:

• Splitting one component's stories across same-title CSF files is a supported pattern (our own docs fixture multiple-csf-files-{a,b}.stories.ts uses it), and sandbox builds now show this warning for it. That is deliberate and truthful: even with the same component in both files, the non-winning file's stories are missing from story-docs today. The message states the unconditional consequence (stories left out) and scopes the rename advice to the case where the files document different components.
• The warning is the visibility half of SB-1847. The real fix - letting the selection represent multiple components per componentId - changes the payload and manifest shapes and deserves its own design discussion; I would rather ship the warning now than hold it hostage to that.

In the field this hit 2 of 22 repos in the Angular QA sweep, and badly: alauda/ui has 25 of 37 titles spanning multiple CSF files, losing 65% of its story snippets with zero warnings anywhere.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

The tests were written first (red on the unpatched selection): warning content including the winner, no warning for single-file components, attached-docs import paths not counted as colliding story files, a byte-identical sorted message for the same collision regardless of entry order (the dedup key), and a fresh message when a file joins the collision or the winner changes.

Manual testing

1. Generate a sandbox: yarn task sandbox --template angular-vite/docgen-server-ts --start-from auto.
2. In the sandbox, copy src/stories/button.stories.ts to src/stories/button-secondary.stories.ts, keep title: 'Example/Button', and point component at HeaderComponent (rename the story exports so story ids stay unique).
3. Run yarn build-storybook and find the warning above in the log; confirm storybook-static/services/core/docgen/example-button.json documents only one of the two components.
4. Give the copy a unique title, rebuild, and confirm the warning is gone and both example-button and the new id have full docgen payloads.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts

• Declare whether manual QA will be needed for this PR during the next release, through qa:needed or qa:skip

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

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Walkthrough

The component entry selector now detects distinct story files that share a component ID and emits deterministic warnings. Tests cover warning content, suppression cases, ordering, and winner changes.

Changes

Component ID Collision Warnings

Layer / File(s)	Summary
Collision detection and warning code/core/src/common/utils/select-component-entry.ts	The selector tracks story import paths by component ID and logs one deterministic warning for distinct collisions. Story selection and attached-doc handling remain unchanged.
Collision warning validation code/core/src/common/utils/select-component-entry.test.ts	Tests mock storybook/internal/node-logger and validate warning content, same-file and attached-doc suppression, deterministic ordering, and message changes when files or winners change.

Merge Risk: 🔵 Low · up to c2f11

The PR only adds a deduplicated warning for story-file collisions and does not change the underlying selection behavior. It is mergeable with explicit owner awareness that the associated test setup should follow the repository’s standard spy and per-test isolation pattern.

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Inline comments:
In `@code/core/src/common/utils/select-component-entry.test.ts`:
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