feat: generate rust-project.json for LSP

Add a CMake module that writes rust-project.json on every configure so
rust-analyzer can resolve the no-Cargo crate layout, sysroot crates,
and proc-macro attributes without Cargo.

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

AGENTS.md

@@ -19,6 +19,7 @@ The project contains:
   entry helpers
 - `macros/src/lib.rs`: host-side `#[rslibc_macros::main]` attribute macro
 - `runtime/main.rs`: minimal example entry point
+- `cmake/RustLSP.cmake`: generates `rust-project.json` for rust-analyzer
 - `CMakeLists.txt`: direct `rustc` library, macro, and executable commands
 
 ## No-Std Design
@@ -112,6 +113,19 @@ Run it on Windows:
 - `RelWithDebInfo`: `-C opt-level=3 -C debuginfo=2`
 - Other or unset configurations: `-C opt-level=0 -C debuginfo=2`
 
+## LSP
+
+rust-analyzer works without Cargo through a generated manifest:
+
+- `cmake/RustLSP.cmake` runs on every configure and writes
+  `rust-project.json` (git-ignored) describing the three crates and the
+  toolchain sysroot, so `core`, `alloc`, and proc-macro attributes resolve.
+- Requires the rustup components:
+  `rustup component add rust-analyzer rust-src`.
+- Open the repository root in the editor so rust-analyzer picks up
+  `rust-project.json`. Re-run CMake configure after changing the crate
+  layout or switching toolchains.
+
 ## Verification
 
 After changing the source or build configuration, run a clean release build