Prepare 1.0.0-alpha.1 release

- CHANGELOG: document the 1.0 framework rewrite
- CI: build Python wheels only on release tags + manual dispatch,
  not on every push/PR

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/python.yml

@@ -2,11 +2,13 @@
 #    maturin generate-ci github -m crates/vtracer-py/Cargo.toml
 name: Python
 
+# Wheel builds are heavy (full platform matrix), so they run only on release
+# tags and on-demand — not on every push/PR. Rust/wasm/Node CI (rust.yml) still
+# gates ordinary commits.
 on:
   push:
     tags:
       - '*'
-  pull_request:
   workflow_dispatch:
 
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