chore: restore mark-stable script on the block facade

The "Mark Platforma Block as Stable" workflow fails with
`ERR_PNPM_NO_SCRIPT  Missing script: mark-stable`: the reusable
`milaboratory/github-ci/.github/workflows/block-mark-stable.yaml@v4`
does `cd "${BLOCK_PATH}"` (default `block`) and then
`pnpm run mark-stable`, but the script was dropped from
`block/package.json` during the project-template migration.

Restore it with the same registry coordinates `prepublishOnly` uses.
The structurer currently strips this script again on
`block-tools structure refresh`; a companion change in `block-tools`
removes that `removeScript` call.

Empty changeset: the published block content is unchanged — this is a
repo-side CI script only.

Author: AStaroverov

.changeset/lucky-pandas-return.md

@@ -0,0 +1,2 @@
+---
+---

block/package.json

@@ -18,6 +18,7 @@
   },
   "scripts": {
     "build": "ts-builder build --target block-facade && block-tools pack",
+    "mark-stable": "block-tools mark-stable -r 's3://milab-euce1-prod-pkgs-s3-block-registry/pub/releases/?region=eu-central-1'",
     "prepublishOnly": "block-tools publish -r s3://milab-euce1-prod-pkgs-s3-block-registry/pub/releases/?region=eu-central-1 --registry-serve-url https://blocks.pl-open.science",
     "do-pack": "shx rm -f package.tgz && pnpm pack && shx mv *.tgz package.tgz",
     "check": "ts-builder type-check --target block-facade"