feat(popover): scaffold 2nd-gen component (phase 2 setup)

[rubencarvalho]

Description

Phase 2 (Setup) of the Popover 1st-gen → 2nd-gen migration (Epic SWC-1993). Scaffolds the core + SWC structure so the component builds and registers — no behavior yet.

• core: PopoverBase property-surface stub, Popover.types (re-exports Placement from the PlacementController), and resolve-trigger / dismissible-stack util stubs wired into utils/index + package.json
• swc: Popover render stub (branches <div popover="auto"> for non-modal vs <dialog> for modal on the modal attribute), popover.css stub, swc-popover element registration, and empty-but-valid stories/ + test/ scaffolds (3/3 pass in the vitest storybook project)

Motivation and context

First implementation phase after the migration plan. The component build auto-discovers via glob and core exposes per-component package.json exports, so no central manifest changes were needed. See CONTRIBUTOR-DOCS/03_project-planning/03_components/popover/migration-plan.md.

Related issue(s)

• Epic SWC-1993

Notes

• Targets the ruben/popover-migration integration branch, not main.
• Opened as draft (WIP); mark ready when reviewed.

Accessibility testing checklist

Scaffolding only — no interactive behavior is wired yet (the host renders a closed popover).

• Keyboard — no focusable behavior in this phase; nothing to trap or activate. Keyboard handling lands in Phase 4 (a11y).
• Screen reader — no roles, names, or ARIA relationships are asserted yet; trigger-side ARIA wiring lands in Phase 4. Full a11y testing will be documented in the a11y and testing phases.

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Latest commit: ffc865a

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[github-actions]

📚 Branch Preview Links

• Documentation Site (first-gen)
• Storybook (first-gen)
• Storybook (second-gen)

🔍 First Generation Visual Regression Test Results

When a visual regression test fails (or has previously failed while working on this branch), its results can be found in the following URLs:

• Spectrum | Light | Medium | LTR
• Spectrum | Dark | Large | RTL
• Express | Light | Medium | LTR
• Express | Dark | Large | RTL
• Spectrum-two | Light | Medium | LTR
• Spectrum-two | Dark | Large | RTL
• High Contrast Mode | Medium | LTR

Deployed to Azure Blob Storage: pr-6353

If the changes are expected, update the current_golden_images_cache hash in the circleci config to accept the new images. Instructions are included in that file.
If the changes are unexpected, you can investigate the cause of the differences and update the code accordingly.

[rubencarvalho]

Consolidated into #6354 (scaffold + API contract). #6354's diff already contained the Setup commit, so review the foundation there. The setup branch remains in the chain.