Add Vue 3 rollout flag for the CI/CD catalog page

Description

Adds the vue3_migrate_ci_catalog feature flag and a vue3_migration.yml file for the CI/CD catalog page. This opts the page into the Vue 3 migration with Option 1. The bundler builds both a Vue 2 and a Vue 3 chunk. Rails serves the Vue 3 chunk only when the flag is on. The flag is default-off.

The page mounts one app, GlobalCatalog, on #js-ci-cd-catalog. It reaches 135 .vue files. 31 are specific to this page, and 28 of those are in ci/catalog. Only 6% of that tree already runs under Vue 3, so most of this code runs on @vue/compat for the first time.

How to verify

1. Check out this branch, then run gdk restart.
2. Seed some catalog content, otherwise both runtimes render an identical empty page and prove nothing:

   bundle exec rake "gitlab:seed:ci_catalog_resources[flightjs, 25]"

   The seeder does not create releases, so every resource shows as Unreleased. To exercise the version selector and the Components tab you also need a release per project. See #616347.
3. Feature.enable(:vue3_migrate_ci_catalog), then load /explore/catalog.

Confirm the Vue 3 bundle is really serving before comparing anything. Without this check, two identical screenshots prove nothing.

Signal	Flag off	Flag on
Entry chunk in the page HTML	pages.explore.catalog.js	pages.explore.catalog.vue3.js
document.querySelectorAll('[data-gitlab-vue3-app]')	empty	GlobalCatalog, CustomBreadcrumbsRoot
Console	no notice	[gitlab] [V] Using Vue.js 3 for GlobalCatalog [gitlab] [V] Using Vue.js 3 (with @vue/compat) for CustomBreadcrumbsRoot

Both the main app and the breadcrumb app run under Vue 3.

Screenshots

Same GDK, same seeded data, only the feature flag differs.

Catalog list, 24 seeded resources:

Before (Vue 2)	After (Vue 3)

Resource detail page, showing the version selector, the include: snippet, the Inputs table and the "On this page" navigation:

Before (Vue 2)	After (Vue 3)

After selecting the non-latest version v1.0.0 from the version dropdown:

Before (Vue 2)	After (Vue 3)

The list and version-selected pairs are byte-identical. The detail pair differs only by a maximum of 1/255 on a single channel, in the empty area below the content, which is rendering noise rather than a visible change.

Manual checks

The catalog was reviewed for the patterns that @vue/compat changes. Each row was exercised with the flag off and then on.

Area	Why it is a risk	Result
Version selector	v-model and @select on the same GlCollapsibleListbox; value/input becomes modelValue/update:modelValue	Pass. Opening, searching, and selecting v1.0.0 updated the toggle text, the version badge, the ?version= query and the include: snippet identically in both runtimes.
Tabs	GlTabs enumerates child vnodes; tabs are lazy and sync with query params	Pass. Components and Readme both render.
Breadcrumb	A second Vue app sharing the same router instance	Pass. Updates to the resource name on navigation.
Components tab	Inputs table and the "On this page" sidebar	Pass. Table and sidebar render with correct values.
List rendering	Avatars, version badges, descriptions, tab counters, keyset pagination	Pass. 24 resources, matching tab counts.

Not exercised in this round, and worth a look before the flag ramps past gitlab-org: the filtered-search tokens, the directive-based tooltips, the list skeleton loader, README GFM rendering, row click-through, and the "Components:" name series.

Automated checks

• bundle exec rspec spec/lib/gitlab/vue3_migration_files_spec.rb — 539 examples, 0 failures
• node scripts/frontend/vue3_migration_stats.mjs --owners reports the page as rollout under vue3_migrate_ci_catalog

Follow-ups found while verifying

• #616347 — the catalog seeder creates no releases, so a seeded GDK cannot exercise the version selector, the Components tab or the Readme tab.
• #616349 — the version selector keeps two sources of truth. selectedVersionId is written by v-model but read by nothing, so a v-model breakage would be almost silent.

References

• Migration issue: #616161
• Rollout issue: #616162
• Epic: &23285