Add reactivated_notation alias and Within-parity specs

- reactivated_notation(date) aliases activated_notation for
  self-documenting call sites when re-anchoring a satisfied Interval
- Specs verify Interval computes final_date/start_date identically
  to Within, documenting the conceptual relationship

Author: rymiwe

lib/sof/cycle.rb

@@ -217,7 +217,8 @@ def initialize(notation, parser: Parser.new(notation))
 
     attr_reader :parser
 
-    delegate [:activated_notation, :volume, :from, :from_date, :time_span, :period,
+    delegate [:activated_notation, :reactivated_notation, :volume, :from,
+      :from_date, :time_span, :period,
       :humanized_period, :period_key, :active?] => :@parser
     delegate [:kind, :recurring?, :volume_only?, :valid_periods] => "self.class"
     delegate [:period_count, :duration] => :time_span

lib/sof/parser.rb

@@ -66,6 +66,8 @@ def activated_notation(date)
       self.class.load(to_h.merge(from_date: date.to_date)).notation
     end
 
+    alias_method :reactivated_notation, :activated_notation
+
     def ==(other) = other.to_h == to_h
 
     def to_h

spec/sof/cycles/interval_spec.rb

@@ -24,6 +24,20 @@ module SOF
       end
     end
 
+    describe "relationship to Within" do
+      # Interval uses the same window computation as Within but with
+      # different lifecycle semantics (repeating vs one-shot)
+      let(:within) { Cycle.for("V1W24MF#{from_date}") }
+
+      it "computes final_date the same as Within" do
+        expect(cycle.final_date).to eq within.final_date
+      end
+
+      it "computes start_date the same as Within" do
+        expect(cycle.start_date).to eq within.start_date
+      end
+    end
+
     @from = "2026-03-31".to_date.to_fs(:american)
     it_behaves_like "#to_s returns",
       "1x every 24 months from #{@from}"
@@ -146,6 +160,18 @@ module SOF
       end
     end
 
+    describe "#reactivated_notation" do
+      it "returns a new notation with the updated from_date" do
+        new_notation = cycle.reactivated_notation("2028-03-15".to_date)
+        expect(new_notation).to eq "V1I24MF2028-03-15"
+      end
+
+      it "is functionally identical to activated_notation" do
+        date = "2028-06-01".to_date
+        expect(cycle.reactivated_notation(date)).to eq cycle.activated_notation(date)
+      end
+    end
+
     describe "activation" do
       let(:notation) { "V1I24M" }