feat(ep-commerce): publish $ctx.productExtensions from product provider (ADR-0007)

[field123]

What

Publishes the current product's extensions as a flat, null-safe, slug-keyed map on $ctx.productExtensions from EPProductProvider, so designers and custom components bind to curated named fields — e.g. $ctx.productExtensions['products(iso-standard)'].reference — instead of raw $ctx.currentProduct.rawData.data.attributes.extensions[...] chains.

Why

Companion to the field-display components (#333/#339): those insulate rendered fields; this insulates bindable values (component props, hrefs) that a rendering component can't feed. It's the visual-builder "curated named data source" pattern — Plasmic's native equivalent is a DataProvider whose fields are discoverable in the data picker.

Changes

• utils/extensions-map.ts — buildExtensionsMap(templates) flattens to { slug: { key: value } }, keyed by the raw template slug (same value the field components' template prop uses) and exposing raw values. A Proxy returns a frozen {} for absent slugs so a binding can never throw — the only missing-slug-safe path inside set-data-cond, which rejects ?.. (Verified @plasmicapp/host's DataProvider keeps data by reference, so the Proxy survives at runtime; ownKeys passes through so picker discovery still works.)
• EPProductProvider publishes $ctx.productExtensions alongside currentProduct, with a canvas mock-swap mirroring EPProductExtensionsProvider so design-time preview stays populated.
• Relocation — the pure extension primitives move out of product-extensions/composable/ into shared lower layers so product/ depends downward, not up: format.ts → utils/field-format.ts, Extension* types → types/extensions.ts, mocks → utils/extensions-mock.ts. All internal importers updated.

Tests

utils/__tests__/extensions-map.test.ts — 5/5 passing (root jest): raw-slug keying, raw values, absent-slug {}, absent-field undefined, empty-templates.

Notes

• Additive & backward-compatible — nothing existing changes; cost is one memoized object walk per product, shipped to every storefront on the package.
• Full design rationale + alternatives in ADR-0007 (iso-storefront repo).