[2026-07] Fix preact entry points; validate package.json integrity in CI (publint + attw)

[henryStelle]

What

• Add a typesVersions entry for ./preact so @shopify/ui-extensions/preact types resolve under moduleResolution: "node" (node10), matching the existing checkout/preact and customer-account/preact entries.
• Remove the dead ./point-of-sale/preact export, which points at files that are never built or published.
• Add publint + attw as devDependencies with package:publint / package:attw scripts and run them in CI after the build, with attw ignoring the pre-existing, harmless false-cjs rule (ESM types declared as .d.ts instead of .d.mts).

Why

import "@shopify/ui-extensions/preact" is required for signal-driven re-rendering, but its types don't resolve under node10 module resolution, which the generated extension templates use. The new CI checks fail with these fixes reverted and pass with them applied, preventing both classes of regression (unshipped entry-point files and missing typesVersions parity).

Backport of #4645 to 2026-07.

[github-actions]

This PR targets a stable release branch (2026-07). Once merged, the change typically also needs to be forward-ported to 2026-10-rc so it ships in the next release.

When you open the forward-port PR, include a line like this in its body so the needs-rc-port label gets removed automatically when that PR merges:

Forward-port of #4649

Accepted formats (comma-separated for multiple):

• #4649
• GH-4649
• 4649
• https://github.com/Shopify/ui-extensions/pull/4649

If a forward-port isn't needed (e.g., the change is stable-only), you can remove the needs-rc-port label manually.