Add module contracts and registry validation

docs/internal/briefing.md

@@ -21,6 +21,8 @@ Outputs:
 
 - `briefing.Package` with common metadata and one report-specific content
   object for Daily, 3-Day, Weekend, or Storm Report
+- module registry definitions for known module IDs, stanza names, option
+  shapes, fact requirements, report compatibility, and missing-data behavior
 - optional `currentConditions` prompt context from normalized
   `/conditions/current` data when available
 - optional structured `weatherStory` context on report-specific briefing
@@ -30,6 +32,8 @@ Outputs:
 ## Boundaries
 
 - This package selects and shapes weather facts for prompts.
+- It owns module registry validation, but app orchestration does not execute
+  modules yet.
 - It does not fetch weather data, compare prior snapshots, build
   `data_package` files, invoke Scriptorium, or write workflow metadata.
 
@@ -65,6 +69,10 @@ None. Builders either return a complete briefing package or an error.
   least one derived summary.
 - Storm briefing construction requires a Storm Report definition and forecast
   bundle.
+- Module registry construction rejects duplicate module IDs and duplicate
+  stanza names.
+- Module composition validation rejects unknown modules, duplicate modules,
+  incompatible report/module combinations, and invalid typed options.
 - Save failures include path and operation context.
 
 ## Tests
@@ -75,6 +83,7 @@ Inspect:
 - `internal/briefing/three_day_test.go`
 - `internal/briefing/weekend_test.go`
 - `internal/briefing/storm_test.go`
+- `internal/briefing/modules_test.go`
 - `internal/app/app_test.go`
 
 ## Invariants