Add 3-Day Outlook generation

docs/internal/changes.md

@@ -11,8 +11,8 @@ produces compact change records for prompt input data packages.
 
 Inputs:
 
-- prior Daily briefing package
-- current Daily briefing package
+- prior briefing package
+- current briefing package
 - configured Recent Changes thresholds
 
 Output:
@@ -37,7 +37,7 @@ The app maps these config fields into comparison thresholds:
 
 ## Behavior
 
-Daily comparison currently detects:
+Daily and 3-Day comparison currently detect:
 
 - temperature changes crossing configured thresholds
 - precipitation probability and timing changes
@@ -47,17 +47,20 @@ Daily comparison currently detects:
 
 When no prior comparable snapshot exists, the app sends an empty Recent Changes
 section in the data package. Daily Today and Daily Tomorrow are compatible for
-same-valid-date comparison through the report registry.
+same-valid-date comparison through the report registry. 3-Day Outlook compares
+with prior 3-Day Outlook snapshots for the same valid local date.
 
 ## Failure Behavior
 
-Daily comparison requires both inputs to contain Daily briefing content.
+Daily comparison requires both inputs to contain Daily briefing content. 3-Day
+comparison requires both inputs to contain 3-Day briefing content.
 
 ## Tests
 
 Inspect:
 
 - `internal/changes/daily_test.go`
+- `internal/changes/three_day_test.go`
 - `internal/app/app_test.go`
 
 ## Invariants