chore(skill): close create-core-component gaps surfaced by Date Input

[im-shiv]

Description

Closes gaps in the create-core-component skill that were surfaced while building and reviewing the Date Input component (PR #1881). Every defect found in that component traced back to a missing or actively-misleading instruction in the skill — this PR updates the skill (workflow, references, validator) so future components don't repeat them.

Gaps identified & addressed

1. Runtime view-JS override contract was missing / wrong (root cause of most runtime bugs)

• The accessibility checklist instructed overriding updateValidity/updateEnabled/updateReadOnly to only set ARIA/disabled, with no mention of calling super — which silently drops the base behaviour (data-cmp-* attributes, error-message rendering, --filled/--empty).
• Added references/runtime-view-js.md documenting the full FormFieldBase override contract: call super.<handler>(value, state) first; an overridden updateValue must call updateEmptyStatus(); setModelValue is not echoed back to the originating field.
• Rewrote the checklist's "JS View Methods" table and SKILL.md Phase 4 accordingly.

2. No guidance for composite / split widgets

• The focus-clobber bug (typing 2024 → 024) and stray submitted sub-field params came from having no pattern for multi-input widgets.
• Added a "Composite / split widget" base-class category and a dedicated section: one hidden combined input carries the value + name; never put name on visible sub-inputs; never repopulate a sub-input the user is actively editing.

3. Cypress tests were entirely absent from the skill

• The skill only covered unit tests + exporter JSON, so HTL/view-JS/dialog/sync bugs were never caught.
• Added references/cypress-tests.md (required runtime + authoring specs, IT content, formsConstants.js entry, runtime-all embed) plus the two Cypress gotchas: hidden inputs fail be.visible; chained should('(not.)have.attr') breaks because the first yields the attribute value.

4. Broken aria-labelledby pattern

• The shared label template emits no id, so aria-labelledby="...__label" references a non-existent element. Documented; grouped widgets must use aria-label.

5. BEM modifier classes with no CSS

• hideTitleDate shipped a --hidden class that had no styling (dead feature). Added the rule: every emitted modifier must have a matching CSS rule; never ship an empty design dialog.

6. Editor-JS safety & i18n

• Use textContent not innerHTML; don't wrap locale-neutral format strings in Granite.I18n.get; wrap hard-coded default UI text with @ i18n.

7. Incomplete/incorrect file checklist (component-anatomy.md)

• Was missing the Cypress specs, _cq_styleConfig, the runtime-all embed and the formsConstants.js entry; had the wrong IT-content path and listed .less instead of .css. Corrected.

8. Validator (scripts/validate_component.py) extended with 4 new check categories

• view-JS override contract (super on update*, updateEmptyStatus in updateValue)
• editor-JS innerHTML usage
• BEM modifier classes have a CSS rule + empty-design-dialog warning
• Cypress specs / formsConstants entry / runtime-all embed existence + Cypress-gotcha lints

Types of changes

• Skill / tooling improvement (no runtime component code changed)

[armaang1729]

LGTM, Thanks for updating these skills.