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.

.env.example

@@ -45,6 +45,6 @@ NODE_ENV=
 
 # ── Outillage dev (facultatif) ─────────────────────────────────────────────
 # Override du chemin local du polyfill @ng-eventually/sdk pour `pnpm run
-# link:polyfill` (lien local réactif). Défaut = ../nextgraph/ng-eventually-js/packages/sdk.
+# overlay:polyfill` (overlay local réactif). Défaut = ../nextgraph/ng-eventually-js/packages/sdk.
 NG_EVENTUALLY_LOCAL=