style(menu): conform to code style guides

[Rajdeepc]

Summary

Conformance pass for swc-menu against the TypeScript, CSS, testing, and Storybook style guides in CONTRIBUTOR-DOCS.

• Menu.ts: removed the PlacementController mention from the concrete-class JSDoc. Per 11_base-vs-concrete.md, internal implementation classes belong in base-class (contributor-facing) JSDoc, not concrete-class JSDoc, which is consumer- and CEM-facing. Popover.ts already follows this.
• menu.stories.ts: moved the shared defaultItems template (used by 7 stories) into its own HELPERS section instead of leaving it under PLAYGROUND STORY, matching the documented section order and popover.stories.ts's own layout.
• menu.stories.ts: added the missing TriggerElement.storyName = 'Trigger element'. Without it Storybook renders the title as "Trigger Element" (title case), mismatching menu.mdx's ### Trigger element heading. Both popover.stories.ts and tooltip.stories.ts set this on their own TriggerElement stories.
• menu.test.ts: added descriptive messages to the specific ARIA-attribute and focus-target assertions, matching the sub-pattern already used in popover.test.ts.

menu.css and menu.a11y.spec.ts were reviewed in full and found already conformant; no changes were needed there.

This branch was rebuilt on top of rajdeep/menu-migration after #6763 merged, so it now contains only the conformance commit. The earlier revision carried a stale copy of the test files that predated #6763's review fixes.

Potential guideline improvements

Each item below is a case where Menu's code is correct and consistent with real precedent, but the written guide doesn't describe the pattern:

• CONTRIBUTOR-DOCS/02_style-guide/02_typescript/06_method-patterns.md — the documented method-ordering rule (public first, protected second, private last) doesn't describe the actual, consistent pattern used by complex, controller-composing base classes. Menu.base.ts and Popover.base.ts both group methods by topic (trigger wiring, show/hide, placement) rather than strict access-level, with lifecycle methods (connectedCallback/disconnectedCallback) positioned mid-file per Lit lifecycle order rather than always first/last by visibility. A note describing this topic-grouping exception for complex base classes would prevent a future reviewer from "fixing" working code into a worse, precedent-diverging state.
• CONTRIBUTOR-DOCS/02_style-guide/02_typescript/01_file-organization.md — the "SWC index.ts" example embeds defineElement/global-typemap registration directly inside index.ts, but every checked component (Badge, Popover, Menu) actually uses a separate swc-<name>.ts file for registration, with index.ts as a pure export * from './Component.js'; re-export. The example should be updated to match the real, universal convention.
• CONTRIBUTOR-DOCS/02_style-guide/04_testing/02_storybook-testing.md — the "organize tests into exactly five sections" rule (Defaults, Properties/Attributes, Slots, Variants/States, Dev mode warnings) doesn't match the real pattern for complex, behavior-heavy components. popover.test.ts uses ~19 custom section headers, tooltip.test.ts uses ~12, and menu.test.ts uses 15 (several names, e.g. "Shadow root scoping" and "PlacementController integration", are verbatim-identical across components), confirming this is a deliberate, consistent, cross-component pattern the guide doesn't document. The five-section rule should be scoped to simple components, with guidance for complex ones to use as many named sections as their behavior surface warrants.

Test plan

• yarn eslint clean on all touched files
• yarn prettier --check clean on all touched files
• stylelint clean on menu.css (no changes, verified conformant)
• yarn lint:ai and yarn lint:docs-pages clean
• vitest --run --project storybook components/menu/ — 38/38 tests pass (3 consecutive runs)
• playwright --config=playwright.a11y.2ndgen.config.ts menu.a11y.spec.ts — 6/6 pass

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[caseyisonit]

i dont see a reason not to include the expect messages so it helps with debugging if that were to ever throw an error. I would add them since its only two in the entire file that doesnt follow the pattern.