Use the X.Y.0 release as the gem-version baseline

stdgems.org's "X.Y" key tracks the latest 4.0.x patch level, so once a
gem gets bumped in a Ruby patch release the diff baseline shifts forward
and history that should appear in NEWS.md for the next minor disappears.
Prefer the explicit "X.Y.0" key when present so the baseline always
points at the original X.Y.0 release; fall back to "X.Y" or the highest
matching patch otherwise.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hsbt

tool/update-NEWS-github-release.rb

@@ -24,6 +24,11 @@ def fetch_default_gems_versions(ruby_version)
   json = JSON.parse(body)
   gems = json["gems"] || []
 
+  # Prefer the initial release key (e.g. "4.0.0") over the rolling
+  # major.minor key (e.g. "4.0") so the diff baseline reflects the original
+  # X.Y.0 release rather than the latest patch level.
+  initial_release_key = (ruby_version =~ /\A\d+\.\d+\z/) ? "#{ruby_version}.0" : nil
+
   map = {}
   gems.each do |g|
     # Only include default gems (skip ones marked removed)
@@ -38,7 +43,9 @@ def fetch_default_gems_versions(ruby_version)
       category_versions = versions[category] || {}
       next if selected_version
 
-      if category_versions.key?(ruby_version)
+      if initial_release_key && category_versions.key?(initial_release_key)
+        selected_version = category_versions[initial_release_key]
+      elsif category_versions.key?(ruby_version)
         selected_version = category_versions[ruby_version]
       else
         # Fall back to the highest patch version matching the given major.minor