chore(react): drop migration scaffolding before merging to main

Remove the one-off migration aids that have no value once the v1.0 migration
lands on main:
- scripts/migration/niivue-v1-scan.mjs: a read-only old-API scanner whose only
  job was tracking migration progress ("when it prints nothing, the source is
  clean") - it permanently reports nothing post-merge. Nothing in CI or the
  package scripts invokes it.
- docs/niivue-v1-migration.md: the phased-plan / status / scanner-usage guide,
  tied to the migration moment (and the only file in docs/). The durable
  knowledge lives in the PR history; the changeset still summarizes the break.

Drop the now-dangling doc reference from the changeset. No product code or test
imports either file, so the type-check/build/test gates are unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: korbinian90

.changeset/niivue-v1-core-upgrade.md

@@ -19,7 +19,6 @@ internals moved onto `nv.model`. We adopt the default dual-backend entry, so Web
 used where available (modern PWA browsers) and WebGL2 elsewhere (incl. VS Code webviews).
 
 The migration is centralised in `@niivue/react`; the apps inherit it through that package.
-See `docs/niivue-v1-migration.md` for the full old->new API mapping and the phased plan.
 
 Scene documents (`.nvd`) use niivue v1's native CBOR serialization. On top of that, a JSON
 form of the same v1 document is supported: a JSON `.nvd`/`.nvd.json` can be hand-authored in