Simplify and rationalize the hourly report template

docs/internal/generatedtext.md

@@ -29,14 +29,15 @@ The hourly generated text JSON accepts:
 ```json
 {
   "summary": "string",
-  "timing": "string",
-  "impacts": "string",
+  "forecast_discussion": "string",
+  "precipitation_timing": "string",
   "confidence": "string"
 }
 ```
 
-`summary`, `timing`, and `impacts` are required after trimming whitespace.
-`confidence` is optional and omitted from normalized JSON when blank.
+`summary` and `forecast_discussion` are required after trimming whitespace.
+`precipitation_timing` and `confidence` are optional and omitted from normalized
+JSON when blank.
 
 ## Boundaries
 
@@ -54,7 +55,7 @@ The hourly generated text JSON accepts:
 - Malformed generated-text JSON fails with decode context.
 - Unknown generated-text JSON fields fail during decoding.
 - Empty required hourly fields fail after trimming whitespace.
-- Missing required render-context stanzas fail with the stanza name.
+- Missing optional render-context stanzas become nil module pointers.
 - Invalid render metadata, including missing timezone, missing generated time,
   or invalid valid period, fails before template rendering.