docs: add type-contracts reference with full entity surface and Mermaid diagrams

Comprehensive single source of truth for every named type boundary in the Muse VCS codebase — TypedDicts, dataclasses, Protocols, Enums, and TypeAliases — modelled after the AgentCeption type-contracts reference.

Covers: domain protocol types (SnapshotManifest, DeltaManifest, MergeResult, DriftReport, MuseDomainPlugin), store wire-format and in-memory types (CommitDict/Record, SnapshotDict/Record, TagDict/Record, RemoteCommitPayload), merge engine (MergeStatePayload, MergeState), config (AuthEntry, RemoteEntry, MuseConfig, RemoteConfig), MIDI import (MidiMeta, MusicXMLMeta, NoteEvent, MuseImportData), stash (StashEntry), and error hierarchy (ExitCode, MuseCLIError, RepoNotFoundError).

Includes 8 Mermaid class diagrams covering every entity relationship.