fix: make bench arm bin provisioning idempotent across trials

provisionArmBin symlinks the shell arm's one allow-listed binary into a
per-sitting bin directory, but a sitting's trials share that directory and
re-provision it each trial, so the second trial's symlinkSync threw EEXIST.
This crashed every shell arm (gitea-axi, tea, raw-api) at more than one
trial per sitting; the gitea-mcp arm was unaffected because its shell is
disabled and it symlinks nothing.

Remove any existing link before re-creating it so provisioning is
idempotent. Add a regression test that provisions the same bin directory
twice and asserts it neither throws nor duplicates the link.

bench/guard.ts

@@ -1,4 +1,4 @@
-import { accessSync, constants, mkdirSync, symlinkSync } from "node:fs";
+import { accessSync, constants, mkdirSync, rmSync, symlinkSync } from "node:fs";
 import { join } from "node:path";
 import type { Arm } from "./result.js";
 
@@ -303,7 +303,12 @@ export function provisionArmBin(
       `cannot provision the ${arm} arm: its binary "${binary}" was not found on PATH`,
     );
   }
-  symlinkSync(target, join(binDir, binary));
+  // Idempotent: the trials in one sitting share a single bin directory, so a prior
+  // trial may have already created this link. Remove any existing entry before
+  // re-linking rather than letting symlinkSync fail with EEXIST on the second trial.
+  const linkPath = join(binDir, binary);
+  rmSync(linkPath, { force: true });
+  symlinkSync(target, linkPath);
 }
 
 /** Resolve `binary` to the absolute path of the first executable of that name on PATH. */