🔖 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

.github/workflows/ci.yml

@@ -2,9 +2,9 @@ name: CI
 
 on:
   push:
-    branches: [ main, develop ]
+    branches: [ main, develop, 1.x ]
   pull_request:
-    branches: [ main, develop ]
+    branches: [ main, develop, 1.x ]
 
 jobs:
   verify: