Add 3-Day Outlook generation

docs/internal/forecast-derivation.md

@@ -6,8 +6,8 @@ boundary.
 ## Purpose
 
 `internal/forecast` converts a normalized forecast bundle into inspectable
-daily daypart summaries. These summaries are structured data for later briefing
-builders; they are not rendered report text.
+daily and multi-day daypart summaries. These summaries are structured data for
+later briefing builders; they are not rendered report text.
 
 ## Inputs and Outputs
 
@@ -15,6 +15,7 @@ Inputs:
 
 - `forecast.Bundle`
 - local date and timezone
+- report period, for multi-day summaries
 - configured daypart definitions with `HH:MM` start and end values
 
 Output:
@@ -22,6 +23,8 @@ Output:
 - `forecast.DailySummary` with a civil-day period, daypart summaries, selected
   narrative periods, alert overlaps, discussion context, source warnings, and
   source provenance.
+- `forecast.BuildPeriodDailySummaries` output with one clipped daily summary
+  for each local day or partial day in a report period.
 
 ## Boundaries
 
@@ -41,6 +44,8 @@ Output:
 - Alerts are selected by overlap with the daily period and each daypart.
 - Narrative periods and discussion context are selected as broader source
   context for later briefing builders.
+- Multi-day period summaries clip the first and last local days to the resolved
+  report period before selecting hourly periods and alerts.
 
 ## Failure Behavior