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

@@ -3,11 +3,13 @@
 
 # Cappuccino: Build Desktop-Class Web Applications
 
-> **🛑 Legacy Branch: Node.js Tombstone**
-> This branch is the final, unmaintained state of the pre-Aristo3 Node/npm era (tagged `v1.4.0`), remaining visually and mechanically compatible with the `1.3.1` npm release. Bug fixes and improvements from the main branch are included. It uses Aristo2 as its theme.
+> **✨ Project Status: v1.5.0 Baseline & Upcoming v2.0.0 Toolchain**
 > 
-> While the main branch is intended to maintain backward theme and toolchain compatibility, no guarantees are made that this branch will be advanced in sync with it. This branch is intended solely as a frozen artifact for those requiring an extended transition period to Cappuccino 2.
+> 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?