Add stable identities and a rename-aware type registry #7

Current State

Serializing .NET type full names as identity turns every class rename or move into missing nodes in saved assets. Node types, port types and host functions need opaque persistent identities, and renames must be registry data rather than asset edits. Consumers own their type sets (an engine registers vector and texture types; a video domain registers frame types).

Tasks:

• StableId (type identity) and ElementId (element identity) GUID value types with canonical text form
• IdentityRegistry<T> resolving identities through rename chains, rejecting empty identities, duplicates and rename cycles
• GraphTypeRegistry for node types, port types, host functions and per-node-type migrations
• IGraphTypeModule registration units for generated discovery (Generate node and port type registration at compile time #11)
• Typed host-function registry entries (parameter and result port types)

Acceptance Criteria:

• Renamed identities resolve to the current entry through any chain length
• No API accepts or stores CLR type names as identity
• The rename map is the only mechanism needed for a type rename

Related:

• Epic: MASS4.Graph core — headless model, identity, editing and persistence #1 (core epic)
• Extract stable type identity into a shared library for engine, graphs and DataAssets Turian#331 (shared identity library extraction; this registry is its graph-side consumer)