docs: Establish v1.5.0 baseline and direct legacy users to 1.4.0

Merge Aristo3 theme and make it default.
Update README to reflect transition to Cappuccino 2.
Change README extension from .markdown to .md for universal editor support.

README.md

@@ -3,8 +3,13 @@
 
 # Cappuccino: Build Desktop-Class Web Applications
 
-> **✨ Project Status: Active Development & Node.js Transition**
-> Cappuccino has been under continuous development since 2008 and is actively maintained. A major transition to a modern, **Node.js-based toolchain** has recently been finalized. The current release is a production-ready Release Candidate, with a formal release scheduled for 2026. It is stable, fast, and ready for new projects.
+> **✨ Project Status: v1.5.0 Baseline & Upcoming v2.0.0 Toolchain**
+> 
+> Cappuccino has been under continuous development since 2008 and is actively maintained. This v1.5.0 release establishes the new resolution-independent Aristo3 theme as the baseline. While Aristo2 remains included, using it requires explicitly setting the theme descriptor.
+
+> For users seeking a complication-free alternative who wish to avoid the Aristo3 work entirely, the legacy-1.4.0 branch provides an unambiguous freeze point. Please note, however, that this legacy branch is not guaranteed to receive any future bug fixes or improvements.
+
+> Active development is now focused on the upcoming v2.0.0 release, which will transition the full toolchain to Golang and the platform-native binaries it produces, leaving Node.js and npm behind.
 
 ## Why Use Cappuccino?