Document template partials

docs/internal/reporttemplate.md

@@ -12,6 +12,7 @@ Tomorrow, and Hourly.
 The package embeds assets from:
 
 - `internal/reporttemplate/templates/*.md.tmpl`
+- `internal/reporttemplate/templates/partials/*.md.tmpl`
 - `internal/reporttemplate/schemas/*.schema.json`
 
 Generated-text prompt source files live under
@@ -73,6 +74,12 @@ template and renders from `generatedtext.DailyRenderContext`.
 Templates use `text/template` with `missingkey=error`, so missing context fields
 fail rendering instead of producing incomplete Markdown.
 
+Daily and Tomorrow call the shared `daypart_forecast` partial. Today calls
+`today_daypart_forecast` so it can omit elapsed or missing dayparts. Daily,
+Today, and Tomorrow call the shared `precipitation_timing` partial. Partial
+files are parsed with each top-level template at render time and receive the
+same typed render context as the caller.
+
 ## Schema Contract
 
 The GeneratedText schemas describe the structured prose Scriptorium is expected
@@ -92,6 +99,7 @@ slots consumed by the template.
 - Unknown template IDs return actionable lookup errors.
 - Unknown schema IDs return actionable lookup errors.
 - Template parse errors include the template ID.
+- Partial read or parse errors include the partial path.
 - Template execution errors include the template ID and usually identify the
   missing context field.
 
@@ -107,6 +115,7 @@ Inspect:
 ## Invariants
 
 - Embedded templates and schemas live as separate files, not inline Go strings.
+- Shared Markdown partials live under `templates/partials/`.
 - Report definitions select templates by ID.
 - Templates render from curated render contexts, not raw data packages.
 - GeneratedText schemas describe LLM prose slots, not deterministic weather