Correct daypart identity and display handling

docs/internal/briefing.md

@@ -46,6 +46,12 @@ missing sources, and prompt-safe SPC values omit geometry and other
 template-only or source details. The complete module composition is in
 [module internals](module.md); fact derivation is in [fact contracts](facts.md).
 
+Derived daypart-summary maps use the forecast package's canonical daypart
+identity and reject any collision instead of replacing an earlier value.
+Planning applies the same identity when recognizing morning, afternoon,
+evening, and overnight windows; display labels remain separate and preserve
+configured text with rune-safe first-letter capitalization.
+
 `area_forecast_discussion` accepts an optional typed section filter. Accepted
 typed option pointers are normalized to the declared value type before builder
 execution. Planning modules are report-specific: `daily_planning` supports Daily,