chore: configure shared Cargo build caching

.cargo/config.toml

@@ -0,0 +1,6 @@
+# Keep intermediate build artifacts outside individual worktrees and reuse them
+# across parallel development of this repository. Final artifacts remain in each
+# worktree's target directory.
+[build]
+build-dir = "{cargo-cache-home}/build/yoi"
+incremental = false

devshell.nix

@@ -6,7 +6,11 @@ pkgs.mkShell {
     git
     rustc
     cargo
+    pkgs.sccache
   ];
+  RUSTC_WRAPPER = "${pkgs.sccache}/bin/sccache";
+  # sccache is additive to Cargo's shared build-dir, so keep its disk usage bounded.
+  SCCACHE_CACHE_SIZE = "5G";
   buildInputs = with pkgs; [
     pkg-config
     openssl