feat(menu): scaffold 2nd-gen file structure

[Rajdeepc]

Description

Creates the core and SWC directory structure for Menu in the 2nd-gen package: base classes, concrete components, stub CSS, and swc-<tag>.ts registration entry points for swc-menu, swc-menu-item, swc-menu-group, and swc-menu-separator, bundled together in one components/menu package. Wires the new ./components/menu export into core's package.json. Adds a minimal Playground story for a Storybook smoke test.

MenuItem/MenuGroup/MenuSeparator are bundled into this same pass (rather than scaffolded separately) to match how Tabs (Tab/TabPanel) and Accordion (AccordionItem) structure their composite sub-parts — confirmed against the workstream status table, which tracks "Menu" as a single row with no separate rows for those three units.

No behavior is implemented yet: components extend SpectrumElement directly and render only a slot. Trigger wiring, swc-popover anchoring, ARIA/keyboard, and the full API surface are out of scope for this phase (2nd-gen swc-menu is a full menu-button host per the accessibility/rendering analysis docs — a larger departure from 1st-gen's list-only sp-menu than a typical port, so intentionally deferred rather than guessed at here).

Motivation and context

First step ("Set up file structure in 2nd-gen") of the Menu 1st-gen → 2nd-gen migration, so implementation work (API, a11y, styling) has a structure to build on.

Note for reviewer: no migration-plan.md exists yet for Menu under CONTRIBUTOR-DOCS/03_project-planning/03_components/menu/ — only the accessibility and rendering-and-styling analysis docs, which this scaffolding is based on. Worth closing out before the API migration phase starts.

Related issue(s)

• SWC - 1983

Screenshots (if appropriate)

N/A — no visual output yet beyond an unstyled slot passthrough.

Author's checklist

• I have read the CONTRIBUTING and PULL_REQUESTS documents.
• I have reviewed at the Accessibility Practices for this feature, see: Aria Practices
• I have added automated tests to cover my changes.
• I have included a well-written changeset if my change needs to be published.
• I have included updated documentation if my change required it.

No automated tests, changeset, or documentation were added: this is Phase 2 (file-structure) scaffolding only, with no publishable behavior change. Validation for this PR was yarn build (both packages), eslint/stylelint/prettier, and yarn storybook:build (confirms components-menu--playground renders). Dedicated tests land in the testing phase; docs land in the documentation phase.

Manual review test cases

• Confirm the new components register and render
  1. Run yarn storybook in 2nd-gen/packages/swc
  2. Open the "Menu" story
  3. Expect swc-menu to render its slotted swc-menu-item/swc-menu-group/swc-menu-separator children with no console errors

Device review

• Did it pass in Desktop?
• Did it pass in (emulated) Mobile?
• Did it pass in (emulated) iPad?

Accessibility testing checklist

• Keyboard — Not applicable yet. These components implement no interactive behavior in this PR (no trigger, no roving tabindex, no ARIA) — they are unstyled slot passthroughs pending the API and accessibility migration phases. Confirm there are no regressions elsewhere (these tags are new and not yet consumed by any other component).
• Screen reader — Not applicable yet, for the same reason. No roles, names, or states are set on any of the four elements in this PR; that work is scoped to the accessibility migration phase, per CONTRIBUTOR-DOCS/03_project-planning/03_components/menu/accessibility-migration-analysis.md.

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Latest commit: 081f927

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📚 Branch Preview Links

• Storybook (Gen2, dev mode)
• Storybook (Gen2, prod mode)
• Documentation Site (Gen1)
• Storybook (Gen1)

🔍 Gen1 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-6596

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