perf: Share Cargo build cache across git worktrees (#60)

Use a common target directory for all worktrees to avoid recompiling the backend in every worktree directory.

scripts/cargo.js

@@ -0,0 +1,31 @@
+#!/usr/bin/env node
+/**
+ * Drop-in replacement for the `cargo` binary.
+ * If CARGO_TARGET_DIR is already set we leave it alone.
+ * Otherwise we compute a path that is the same for every git-worktree:
+ *   <main-clone-root>/target
+ *
+ * Usage   :  node scripts/cargo.js <cargo-subcommand> […]
+ * NPM     :  "backend:build": "node scripts/cargo.js build --release …"
+ */
+const { spawnSync, execSync } = require('node:child_process');
+const path = require('node:path');
+
+function sharedTarget() {
+  try {
+    // Works in any work-tree folder
+    const common = execSync('git rev-parse --git-common-dir', { encoding: 'utf8' }).trim();
+    return path.resolve(common, '..', 'target');
+  } catch {
+    return undefined;                 // not a git repo? -> let Cargo use ./target
+  }
+}
+
+const env = { ...process.env };
+if (!env.CARGO_TARGET_DIR) {
+  const dir = sharedTarget();
+  if (dir) env.CARGO_TARGET_DIR = dir;
+}
+
+const result = spawnSync('cargo', process.argv.slice(2), { stdio: 'inherit', env });
+process.exitCode = result.status;