docs(logdb): note immediate-mode persist is O(db-size) per message (P2)

ober

52f89179a81bf58b940e5c896f3e67d72727b274

diff --git a/signal/logdb.ss b/signal/logdb.ss
index 0ad5ea0..8872b03 100644
--- a/signal/logdb.ss
+++ b/signal/logdb.ss
@@ -450,6 +450,12 @@
   (def (maybe-string-length v)
     (if (string? v) (string-length v) 0))
 
+  ;; Persistence policy. Every persist re-serializes the whole SQLite db
+  ;; (sqlite-db->bytevector) and re-AEAD-seals the ENTIRE container, so a persist
+  ;; is O(db-size). 'batch (the default) amortizes that cost over a batch of rows;
+  ;; 'immediate persists on every write and is therefore O(db-size) PER MESSAGE —
+  ;; it is opt-in only (JERBOA_SIGNAL_LOG_PERSIST) and meant for debugging, not
+  ;; steady-state use. Incremental sealing would lift this but is not yet done.
   (def (log-persist-policy)
     (let ([v (getenv "JERBOA_SIGNAL_LOG_PERSIST")])
       (cond