Add a new narrative forecast module and remove the unimplemented daily forecast stub

docs/internal/briefing.md

@@ -27,15 +27,14 @@ Outputs:
 - `ModuleDefinition` values with module ID, stanza name, option type,
   supported reports, fact requirements, missing-data behavior, and builder
 - `module.Output` values for source-oriented stanzas:
-  `metadata`, `current_conditions`, `alert_digest`,
+  `metadata`, `current_conditions`, `narrative_forecast`, `alert_digest`,
   `area_forecast_discussion`, and `weather_story`
 - `module.Output` values for derived stanzas:
   `derived_daily_summary`, `derived_daypart_summaries`, `precip_timing`,
   `outdoor_windows`, and `tomorrow_planning`
 
-The registry also contains accepted composition entries for modules that do not
-emit stanzas until a builder exists. App orchestration skips those entries when
-constructing snapshots.
+Every registered composition entry has a builder. Unknown or unimplemented
+module IDs fail validation instead of being skipped.
 
 ## Boundaries