seed-policies: one failed manifest fetch installs the fallback set as a complete v5, and later runs never repair it #350

At main 9136aa4, scripts/seed-policies.sh can install an incomplete policy set that later runs never repair:

• When the manifest.json fetch fails once (any curl failure, including the 5 s connect timeout), POLICY_FILES becomes FALLBACK_FILES: velstand.onnx, alpha_sitstand.onnx, alpha_ground_pick.onnx, the two kicks, the two rollers and roulade.onnx. It does not include alpha_walking.onnx or alpha_stand.onnx.
• The staged set is then installed with write_source "$staging" "${POLICY_VERSION}", so its .source records version=v5, the same as a complete set.
• On the next run have (5) is not below want (5), so the script exits before fetching anything. The incomplete set stays until POLICY_VERSION moves past it.

Observed with scripts/duck-sim, headless: the first start hit a DNS timeout on the manifest, the fallback set was installed, and the simulator stopped with error: no policy set in …/policies, because the simulator's params name alpha_walking.onnx and alpha_stand.onnx. Running the script again did not repair it; deleting the policies directory by hand did.

On a robot the fallback matches robotd's defaults, so it runs, but the set is still recorded as complete and any policy the manifest lists beyond the fallback stays missing until the next version.

A possible fix: record a fallback install so that it does not satisfy the version check (for example no version= in .source, or a separate marker), so the next run fetches the manifest again.