fix(19.7): RA-IN-01 describe the Redis store's mount-time warm-up correctly

lazyStore's and newStore's doc comments no longer claim that nothing dials
at mount: Mount starts a background warm-up that dials Redis with a backoff
and logs each failed attempt. The comment records the decision to keep the
warm-up for commands that mount pier without serving, such as route:list:
it only dials and logs until the process exits.

modules/pier/pier.go

@@ -150,10 +150,10 @@ func Mount(host Host, app *backpack.App, plugins []party.Plugin) error {
 }
 
 // newStore builds the session store mcp.sessions.store selects: the memory
-// store, subscribed to its own events at once, or a Redis store opened on
+// store, subscribed to its own events at once, or a Redis store opened
-// first use from redis.url under cooler's key prefix (nothing dials at
+// from redis.url under cooler's key prefix by a background warm-up that
-// mount itself; a background warm-up opens it with a backoff, and a missing
+// Mount starts (Mount itself never waits for it); a missing URL is a 503 on
-// URL is a 503 on stateful requests, logged).
+// stateful requests, logged.
 func (p *Pier) newStore() Store {
 	if p.cfg.SessionStore != StoreRedis {
 		ms := newMemoryStore()

modules/pier/store_redis.go

@@ -228,9 +228,14 @@ func (s *redisStore) Subscribe(ctx context.Context, fn func(Event)) error {
 	return nil
 }
 
-// lazyStore opens the Redis store on first use, so nothing dials at mount
+// lazyStore opens the Redis store without blocking Mount: Mount starts warm,
-// (route:list and tests never dial) and a missing redis.url is a 503 on the
+// which dials in the background with a backoff (every failure logged as
-// first request, logged once, not a boot failure (cooler's rule).
+// "session store not open yet, retrying"), and the first request of any
+// path kicks an open too. A missing or unreachable Redis is therefore never
+// a boot failure (cooler's rule): stateful requests answer 503 until the
+// store opens, and the missing redis.url is logged once. A command that
+// mounts pier without serving (route:list) also starts the warm-up; it only
+// dials and logs until the process exits, so it is not skipped there.
 type lazyStore struct {
 	open    func(ctx context.Context) (Store, error)
 	mu      sync.Mutex