Add collected and derived fact contracts

docs/internal/facts.md

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+# Fact Contracts Internals
+
+This document describes the implemented fact contract boundary.
+
+## Purpose
+
+`internal/facts` separates normalized upstream facts collected for a report run
+from conservative report-scoped facts derived from them. The package gives app
+orchestration one place to build reusable facts before briefing construction.
+
+## Inputs And Outputs
+
+Inputs:
+
+- `weatherdata.Bundle` from the Weather API adapter
+- resolved report definition and valid period
+- report timezone
+- configured daypart definitions
+
+Outputs:
+
+- `facts.CollectedFacts` with normalized source facts plus separate source
+  provenance and warnings
+- `facts.DerivedFacts` with valid-period forecast slices, alert overlaps,
+  daily summaries, daypart summaries, and Storm Report window summary
+
+## Boundaries
+
+- This package owns fact assembly and reusable deterministic derivation for a
+  report run.
+- It does not fetch upstream data, build prompt wording, compare prior
+  snapshots, write workflow state, invoke Scriptorium, or define modules.
+
+## Config Fields Used
+
+- `dayparts[].name`
+- `dayparts[].start`
+- `dayparts[].end`
+- `weather_api.timezone`
+
+## External Adapters Used
+
+None directly. Collected facts are built from `weatherdata.Bundle`.
+
+## State Or Manifest Behavior
+
+None. Source provenance and warnings remain data fields for downstream metadata
+and inspection.
+
+## Failure Behavior
+
+- Invalid or missing report valid periods return an error.
+- Invalid timezone names return an error.
+- Missing required hourly forecast data returns the underlying forecast
+  derivation error for reports that require daily summaries.
+- Missing optional narrative, alert, discussion, daily, or weather story data
+  produces empty or nil derived fields.
+
+## Tests
+
+Inspect:
+
+- `internal/facts/facts_test.go`
+- `internal/app/app_test.go`
+
+## Invariants
+
+- Collected facts are built once from a fetched bundle.
+- Derived facts are scoped to one resolved report.
+- Source provenance and warnings stay separate from ordinary fact fields.
+- Prompt-specific wording and one-off presentation decisions stay outside this
+  package.