Correct daypart identity and display handling

docs/config.md

@@ -223,6 +223,13 @@ derivation. Every item needs `name`, `start`, and `end`; start and end use
 (`06:00`–`10:00`), `midday` (`10:00`–`15:00`), `afternoon`
 (`15:00`–`17:00`), and `evening` (`17:00`–`24:00`).
 
+Names remain display text, but each name must have a distinct canonical
+identity. Canonicalization trims whitespace, lowercases letters, and collapses
+punctuation and whitespace to underscores; for example, `Morning`,
+`morning!`, and `morning` conflict. Planning recognizes the canonical
+identities `morning`, `afternoon`, `evening`, and `overnight` regardless of
+their display capitalization or punctuation.
+
 ### `reports`
 
 `reports` optionally overrides a report's ordered deterministic modules and