Assembly definitions

A Unity project compiles into the assemblies Unity would make:
- one per .asmdef, under Assets or in an embedded package, holding its folder's scripts (and .asmref folders), minus subfolders with their own;
- Assembly-CSharp-firstpass for Plugins and Standard Assets;
- Assembly-CSharp for the rest. Firstpass and Assembly-CSharp reference every auto-referenced asmdef. References may name an asmdef or give its GUID; references outside the project (Unity packages) are reported. Editor-only, test and unmet-defineConstraints assemblies are left out, as in a player build. A reference cycle is a compile error.
All assemblies build in one dotnet build of a generated solution, and MSBuild's incremental build skips unchanged ones.
Hot reload works per assembly. A changed assembly reloads together with every assembly that references it, because their code is bound to the old one. Types may move between assemblies, and assemblies may appear or disappear.
ScriptableObjects hot reload too. Live instances are tracked weakly. Each one gets OnDisable, a new instance with its fields copied takes its place (references in scripts, other assets and the asset cache are updated), and then OnEnable runs.
An MCP e2e test covers a Coin Dash copy with asmdefs: a GUID reference, an asmref sharing internals, firstpass, an editor-only assembly and a broken test assembly. It edits a core assembly and checks that its dependents reload with their state and with ScriptableObjects swapped. It also checks that a cycle is a compile error.
Next: MCP resources and the exec tool, .csproj generation for IDEs, precompiled plugin DLLs.