Document profile comparison workflow

docs/internal/prepared-report.md

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+# Prepared Report Internals
+
+`internal/app` builds a `preparedReport` after collection and before profile
+execution. This is the immutable boundary shared by ordinary report generation
+and profile comparison; it is not a durable artifact.
+
+Preparation builds report facts, the configured module snapshot, briefing
+metadata, the curated prompt-input package, serialized YAML, and the
+generated-text definition. It deep-copies mutable facts, snapshots, metadata,
+and data-package bytes before returning them. Consumers receive independent
+copies so one execution cannot change another's input or rendering context.
+
+Single-report generation executes one prepared profile and publishes its
+Markdown. Comparison prepares once, gives every selected profile the same YAML
+bytes, and only then assembles the resulting logical bundle. The prompt-input
+shape is owned by [prompt-input internals](prompt-input.md); profile execution
+semantics are owned by [Promptkit integration](../integrations/promptkit.md).
+
+Preparation failure has no publication side effects. Tests for this boundary
+cover mutation isolation, byte equality, and reuse by both execution paths.