Add overrides attribute to Bundler::Definition

Reserve a slot on Definition for the upcoming Gemfile `override` DSL.
This commit only stores the data; the DSL entry point and the resolver
hookup come in later commits.

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

Author: hsbt

bundler/lib/bundler/definition.rb

@@ -10,13 +10,14 @@ class << self
       attr_accessor :no_lock
     end
 
-    attr_writer :lockfile
+    attr_writer :lockfile, :overrides
 
     attr_reader(
       :dependencies,
       :locked_checksums,
       :locked_deps,
       :locked_gems,
+      :overrides,
       :platforms,
       :ruby_version,
       :lockfile,
@@ -88,6 +89,7 @@ def initialize(lockfile, dependencies, sources, unlock, ruby_version = nil, opti
       @specs           = nil
       @ruby_version    = ruby_version
       @gemfiles        = gemfiles
+      @overrides       = []
 
       @lockfile               = lockfile
       @lockfile_contents      = String.new

spec/bundler/definition_spec.rb

@@ -3,6 +3,24 @@
 require "bundler/definition"
 
 RSpec.describe Bundler::Definition do
+  describe "#overrides" do
+    before do
+      allow(Bundler::SharedHelpers).to receive(:find_gemfile) { bundled_app_gemfile }
+    end
+
+    subject { Bundler::Definition.new(bundled_app_lock, [], Bundler::SourceList.new, {}) }
+
+    it "defaults to an empty array" do
+      expect(subject.overrides).to eq([])
+    end
+
+    it "is writable" do
+      override = Bundler::Override.new("rails", :version, ">= 8.0")
+      subject.overrides = [override]
+      expect(subject.overrides).to eq([override])
+    end
+  end
+
   describe "#lock" do
     before do
       allow(Bundler::SharedHelpers).to receive(:find_gemfile) { bundled_app_gemfile }