🔖 chore: establish the 1.x legacy-decorator maintenance line

The 2.x README tells projects on an oxc-based toolchain to stay on 1.x, but no
such line existed — this branch point was version 0.1.0, so the advice pointed
at nothing installable.

- version 1.0.0
- CI runs on this branch as well as main and develop
- README opens with a maintenance-line notice pointing new projects at 2.x and
  naming the one reason to stay here: oxc does not implement the TC39 standard
  decorator transform, while tsc and esbuild do
- CHANGELOG names the release

No functional change: the code is exactly the state merged as PR #3.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SAcqrz3FcadkYr3xG32CjK

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "cereale",
-  "version": "0.1.0",
+  "version": "1.0.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "cereale",
-      "version": "0.1.0",
+      "version": "1.0.0",
       "license": "MIT",
       "devDependencies": {
         "@eslint/js": "^10.0.1",