engine: vault choice is NOT remembered across launches — restored string path crashes store-path RPC (path->string contract violation), wedging the UI; contradicts the documented "remembered across launches" #6

Context

Cross-platform consistency pass of the v0.2.0 release (Linux arm64 deb on Ubuntu 26.04 VM; the failing code is the shared engine, so all three platforms are affected). The no-vault splash promises "The choice is remembered across launches." — the packaged release does the opposite.

Repro (v0.2.0 Linux deb, deterministic)

1. Fresh install, launch → "No vault selected" (works).
2. Choose Vault… → pick any secrets.ini → vault loads, "Store switched: …" in the status line. At this point ~/.keyward/config.json contains {"store":"/…/demo-vault.ini"} (written by store-open, app/backend.rkt:50).
3. Quit and relaunch.
4. Within seconds-to-minutes a raw "Engine message" dialog appears:

   path->string: contract violation
     expected: path?
     given: "/home/turinglambdaai/keyward-test/vault/demo-vault.ini"
   

5. After Close, the UI stays on "Starting embedded Racket CS…" indefinitely; sidebar shows Products 0 and the Choose Vault… button never returns. The instance is wedged until killed; deleting ~/.keyward/config.json restores the first-launch state (verified recovery path).

Root cause (v0.2.0 tag, app/backend.rkt)

current-store is declared "a path string, or #f" (line 30) and restored as a string from config at startup (line 32 via store-path-default). But store-open puts a path object into the parameter (line 47: (current-store p) where p comes from simple-form-path), and the store-path RPC then unconditionally does (path->string p) (line 42) — a contract violation on the restored-string case, raised inside the RPC thread at startup and surfaced as the raw Engine message. The type of current-store differs depending on whether the vault was opened live or restored.

Suggested fix (two lines, shared engine)

Keep the declared string invariant:

• line 47: (current-store (path->string p))
• line 42: (if p p "")

Everything downstream (store-sections, file->lines, …) already accepts path strings.

Found via the vmpal-consistency three-platform pass; happy to send the short-lived-branch PR immediately if you confirm.