safe-bash: mv -n cannot atomically preserve a concurrent destination through injected filesystems #701

mv -n in published @poe-platform/safe-bash@0.1.500 can overwrite a destination created concurrently after its existence check. The public safe-fs rename operation receives only { signal }, so an injected native filesystem cannot preserve the command's no-clobber intent atomically.

The command implementation in dist/safe-bash/commands/filesystem.js checks maybeStat(target) and then calls fs.rename(source, target, { signal }). Our native default rename is atomic, but adding another destination stat in the adapter cannot close this race.

Public API reproduction (run in a package with safe-bash0.1.500):

import { Shell, agentCommands, createMemoryFileSystem } from '@poe-platform/safe-bash';
const fs = createMemoryFileSystem();
const bytes = (s) => new TextEncoder().encode(s);
await fs.writeFile('/source', bytes('source'));
const injected = new Proxy(fs, {
  get(target, key) {
    if (key === 'rename') return async (from, to, options) => {
      // Deterministically create the destination after mv's final stat.
      await fs.writeFile(to, bytes('concurrent destination'));
      return fs.rename(from, to, options);
    };
    const value = Reflect.get(target, key);
    return typeof value === 'function' ? value.bind(target) : value;
  },
});
const shell = new Shell({ fs: injected }).use(agentCommands());
try {
  const result = await shell.exec('mv -n /source /target');
  console.log(result.exitCode, new TextDecoder().decode(await fs.readFile('/target')));
} finally {
  await shell.dispose();
}

Actual: 0 source, replacing the concurrent destination. Expected: keep concurrent destination and preserve /source.

Please expose and propagate a supported atomic no-replace rename operation/capability (or equivalent public option), with native filesystem admission that refuses the mode when it cannot be guaranteed. Include deterministic concurrent-destination coverage through the public command and filesystem APIs. Default overwriting mv is separate and works with our authoritative transaction; this issue is specifically the no-clobber mode.