docs(architecture): describe background shader preparation at registration

Co-Authored-By: Claude <noreply@anthropic.com>

docs/dev/ARCHITECTURE.md

@@ -67,6 +67,8 @@ flowchart LR
 
 - [`Recompiler.cpp`](../../core/shader/recompiler/Recompiler.cpp) runs the stages in this order. With `ANYPS5_ENABLE_SPIRV_TOOLS`, the SPIR-V is also validated and optimized with SPIRV-Tools.
 - `ShaderRecompiler::Recompile` keeps compiled variants in memory, and `ShaderDiskCache` stores them on disk so later runs reuse them.
+- `Driver::RegisterShader` splits shader preparation in two. `PlanRegistered` runs on the registering guest thread: it validates the registers and entry point, decodes the code and builds the recompile request, so invalid registrations still fail there. The RDNA to IR translation and resource plan (`PrepareShader`) run on a background pool of `hardware_concurrency() / 2` detached threads (at least 2, 64 MiB stacks).
+- The snapshot is published as pending. `SourceHandleFor`, `InvocationFor` and `ShaderPreparationTransaction::Edit` / `Read` wait for the pending job before reading `PreparedShaders`, and rethrow its failure. `APS5_SYNC_SHADER_PREPARE=1` prepares on the registering thread instead.
 
 ### Shader MODE register access