CI: cold-install pnpm-state flake on Namespace runners amplified by success()-gated cache-save

Downstream consumers on Namespace cloud runners intermittently fail CI on lock-changing PRs with a cold-install node_modules/.bin materialization failure (Executable not found in $PATH: vitest/madge), preceded by a determinate-nix daemon self-test error (cannot connect to socket at /nix/var/nix/daemon-socket/socket). Observed on livestorejs/livestore#1397 (effect-utils bump). Same commit reruns rotate which jobs pass/fail → confirmed runner-health flake, not code.

Why it sticks (amplifier)

The generated per-job Save pnpm state step is gated:

if: ${{ success() && steps.restore-pnpm-state.outputs.cache-hit != 'true' }}

On a lock change every job cold-misses the pnpm-state cache. success() = whole-job success, so any job whose install completes but whose later step flakes (or whose daemon self-test fails) never seeds the cache → subsequent reruns keep cold-missing → the flake-prone cold path runs every time until a fully-green run happens by luck.

Two improvements (durable, careful — not urgent)

1. Safe cache seeding: seed the pnpm-state cache when the install itself verifiably completed (node_modules/.bin populated, .modules.yaml present, install-state/projection hashes written) even if a later job step fails — but NOT when the install is incomplete (avoid caching a broken links/). This raises seeding probability → fewer flake-prone cold installs on rerun, without caching corruption. Requires a discrete verify-install step to gate on instead of whole-job success().
2. Daemon/setup retry robustness: the existing run-with-nix-gc-race-retry.sh wraps task runs, not the Nix-daemon self-test / devenv setup phase. Wrap (or retry) the daemon-setup/install phase so a transient daemon-socket failure retries instead of failing the whole job.

Root cause of the flake itself is Namespace/determinate-nix daemon availability (external infra); these two changes reduce its blast radius on downstream CI. Refs: pnpm.nix cold-path ~L455-475 / L575-632; genie CI generator Save pnpm state step. Filed from the livestore#1397 investigation; relates to the effect-utils consumption/prevention story (frequent-bump + canary).

Start with pnpm.nix around the referenced cold-path lines and the genie CI generator's Save pnpm state step; read how the current install and cache-hit conditions are produced. Inspect run-with-nix-gc-race-retry.sh and the devenv setup path for the daemon self-test. Done means incomplete installs cannot seed pnpm state, completed installs can seed it after later failures, and transient daemon-setup failures retry.