docs: note why native CMake Rust is skipped

CMake 4.4's experimental Rust support compiles each source as its own
crate, links libraries into executables as native .lib inputs instead
of --extern, and has no proc-macro crate type. Keep direct rustc.

Co-Authored-By: qwen (lmstudio/qwen3.8-27b@q3_k_xl): documented decision

AGENTS.md

@@ -63,7 +63,13 @@ The project contains:
 ## Build System
 
 CMake is configured with `LANGUAGES NONE` and invokes `rustc` through three
-custom commands:
+custom commands. CMake 4.4's experimental native Rust support
+(`CMAKE_EXPERIMENTAL_RUST`) was evaluated and rejected: it compiles each
+source file as its own crate, so multi-module crates break; it links Rust
+libraries into executables as native `.lib` inputs instead of `--extern`;
+and it has no `proc-macro` crate type. Keep the direct `rustc` commands.
+
+The three custom commands are:
 
 1. Compile `macros/src/lib.rs` as a host procedural macro.
 2. Compile `src/lib.rs` and its modules to `librslibc.rlib`.