feat(ep-commerce): per-variation input rendering via EPVariationCase (#336)

[field123]

Summary

Closes #336.

Adds a per-variation "input pattern" capability to EPVariationPicker so each variation on a product can render with its own input style — primarily so a storefront's PDP can render Language as a native <select> and Format as native radios, while other variations continue to render as the existing card/chip pattern.

The shape is the Discriminated Slot Children pattern, modelled after HTML's <picture>/<source>. Consumers compose:

<EPVariationPicker>
  <EPVariationCase forVariation="Language">
    <EPVariationField field="name" />
    <EPVariationOptionSelect />
  </EPVariationCase>

  <EPVariationCase forVariation="Format">
    <EPVariationField field="name" />
    <EPVariationOptionRadioGroup />
  </EPVariationCase>

  <EPVariationCase>{/* un-keyed = fallback for any other variation */}
    <EPVariationField field="name" />
    <EPVariationOptionList />
  </EPVariationCase>
</EPVariationPicker>

What's new

• EPVariationCase — slot host with optional forVariation prop. Keyed cases render only when iterating their named variation; un-keyed cases act as the fallback for any variation no keyed sibling has claimed.
• EPVariationOptionSelect — atomic native <select> renderer for one variation.
• EPVariationOptionRadioGroup — atomic native radio-group renderer for one variation.
• VariationPickerContext now exposes claimedVariations: ReadonlySet<string> and registerClaim(variationName) => unregister. Keyed EPVariationCase instances self-register on mount; the fallback consults the registry to decide whether to render. No React.Children scanning, so Plasmic's runtime div wrapping doesn't break the filter.
• Picker default children slot now contains a single un-keyed EPVariationCase wrapping the existing Field + EPVariationOptionList. Zero-config N-variation usage is unchanged.

Picker hydration fix

While wiring this I noticed the picker's init useEffect wasn't populating selectedValues when no defaultVariantId resolved, leaving option triggers un-highlighted on first load even though a variant was effectively in use. Fixed in the same effect: falls back to variants[0] and populates selectedValues accordingly. Bundled here because the new EPVariationCase registration would re-trigger the bug in any consumer that adopts the new pattern.

Naming

The component is named EPVariationCase (switch/case analogy: "this is the case for variation X"), matching the existing Variation namespace in this package. Earlier drafts of this PR used EPVariationAxis — that's been renamed to align with the rest of the surface (EPVariationField, EPVariationOptionList, etc.) and to keep "variation" as the single domain term consumers see.

Ordering

Preserved from EP. The picker still iterates product.options[] in EP order; forVariation only decides which subtree renders during each iteration. Reorder in the EP catalog to change display order.

Backwards compatibility

Existing projects using <EPVariationOptionList> directly under the picker continue to work — the new default children content is semantically identical to the prior default content. EPVariationOptionList's selectionMode="dropdown" branch is not removed; new use is steered to EPVariationOptionSelect.

Alternatives considered

Rejected during design grilling:

• selectionModeByVariation: Record<variationName, mode> config prop. Awkward Plasmic Studio authoring (no first-class UI for Record<string, enum>), config split between picker prop and child components, silent failure on typo.
• Per-variation explicit instances with no fallback. Forces explicit declaration of every variation even when overriding one.
• Render-prop children (Shopify Hydrogen VariantSelector shape). Unauthorable in Plasmic Studio.

Test plan

• Build the package: yarn build in plasmicpkgs/commerce-providers/elastic-path — verify no TS errors.
• Drop EPVariationPicker on a Plasmic page with a 2-variation product (e.g. ISO 5495:2005 with Language × Format). Default children renders chip pattern for both variations.
• Add <EPVariationCase forVariation="Language"><EPVariationOptionSelect/></EPVariationCase> and <EPVariationCase forVariation="Format"><EPVariationOptionRadioGroup/></EPVariationCase> inside the picker. Confirm Language renders as <select>, Format as native radios.
• Confirm the picker still emits the same useFormContext field names (variation_<axisId>, ProductVariant).
• Confirm ?variant= URL round-trip works through the new renderers.
• Confirm no infinite-render loops by checking the dev-tools console after selecting an option.
• Confirm zero-config drop-in (no EPVariationCase instances authored) renders the picker with the default fallback chip pattern across N variations.