Name the script for what it does, now that it serves two providers

`link-polyfill.ts` handled one provider. It now handles two through a registry,
one of which is not a polyfill, so both the filename and the `link:` verb said
something false — and a name that is short but wrong misleads more than an opaque
one would.

It becomes `overlay-local-checkout.ts`, which is the operation: it copy-overlays a
local package checkout into node_modules for development. "link" also implied a
symlink, which is precisely what the script exists NOT to do — its own header
spends a paragraph on why.

No behaviour changed: same registry, same copy, same `--once`, same watch loop,
same assertion. Both providers exercised afterwards, each reporting the singletons
it must preserve — one package for the data layer, two for the indexing package,
whose own checkout carries `file:` links and would otherwise smuggle in a second
verifier.

Seven references repaired by rewriting the sentence that carried them rather than
swapping a path, across the script, `package.json`, `.env.example` and four
doctrine leaves. No compatibility alias left behind: an old name kept "just in
case" would restore exactly the ambiguity being removed.

package.json

@@ -17,8 +17,8 @@
     "build:orm": "rdf-orm build --input ./src/shared/shapes/shex --output ./src/shared/shapes/orm",
     "validate": "bun scripts/validate.ts",
     "build:ng": "bash scripts/build-ng-packages.sh",
-    "link:polyfill": "bun scripts/link-polyfill.ts polyfill",
-    "link:indexing": "bun scripts/link-polyfill.ts indexing",
+    "overlay:polyfill": "bun scripts/overlay-local-checkout.ts polyfill",
+    "overlay:indexing": "bun scripts/overlay-local-checkout.ts indexing",
     "storybook": "storybook dev -p 6006",
     "build-storybook": "storybook build"
   },