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

cmake/rust-project.json.in

@@ -0,0 +1,28 @@
+{
+    "sysroot": "@RUST_SYSROOT@",
+    "sysroot_src": "@RUST_SYSROOT_SRC@",
+    "crates": [
+        {
+            "display_name": "rslibc",
+            "root_module": "src/lib.rs",
+            "edition": "2021",
+            "deps": []
+        },
+        {
+            "display_name": "rslibc_macros",
+            "root_module": "macros/src/lib.rs",
+            "edition": "2021",
+            "is_proc_macro": true,
+            "deps": []
+        },
+        {
+            "display_name": "rslibc_example",
+            "root_module": "runtime/main.rs",
+            "edition": "2021",
+            "deps": [
+                { "crate": 0, "name": "rslibc" },
+                { "crate": 1, "name": "rslibc_macros" }
+            ]
+        }
+    ]
+}