chre: #3593376 - Iteration 3: clean up, restructure suites, per-recipe front-end theme tests

Iteration 3 — builds on !1 (merged) and !2 (merged). Cleans up the install file, restructures the test suites, and adds front-end theme coverage across the UI Suite ecosystem plus a runnable example gallery.

What's in this iteration

1. Drop redundant update hooks (refactor) — no pre-release tags, so a fresh install reaches the final state via hook_install; the four hook_update_N (10001-10004) are removed.

2. Restructure the feature suites (test) — shared scenarios move to tests/features/; per-recipe / per-theme folders under tests/features/recipes/ named after the recipe or theme.

3. UI Suite theme matrix (test) — the full Display Builder AI suite runs on six front-end themes, each asserting its own markup via advanced named selectors (tests/selectors/ui-suite-*.json):

• recipes: bootstrap (.navbar), dsfr (.fr-header), base foundation
• themes on display_builder_base (no recipe of their own, pulled + enabled by a THEME CI variable): olivero, daisyui (.navbar), material (.mdc-top-app-bar), uswds (.usa-banner)

Shared features carry no @recipe tag; each recipe/theme feature is @recipe @<name>, so a job runs the shared scenarios + its own folder via not @recipe or @<name>. Theme jobs drop @ai (kept on the three recipe jobs).

4. Example gallery (test) — tests/features/showcase/, tagged @showcase @ai and excluded from the blocking jobs (run on demand with --tags "@showcase"):

• page-examples.feature — the ten real pages from the manual (landing, pricing, about, contact, blog, dashboard, profile, sign-up, 404, UI Suite campaign)
• component-examples.feature — twelve universal components (accordion, alert, badge, button, card, carousel, hero, list, progress, steps, tabs, modal)

Checkpoints

• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Documentation
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release

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