Add Fase 0 telemetry for the server-client migration baseline

Adds dev-only counters, with no behavior change, to answer the open
questions from the migration plan before touching the transport:

- frontend.state/get-invoke-stats: per-thread-api-method count/total-ms/
  max-ms for every <invoke-db-worker call, to census how many invokes a
  typical navigation generates and their latency distribution.
- db-worker-node: per-repo in-flight /v1/invoke counter that logs a new
  concurrency high-water mark, to check whether concurrent client writes
  actually overlap before deciding on a write queue.
- subs-loader/get-stats: dedup-hit vs new-entry counts for load!, and
  batch-size distribution per slot kind at flush, to check whether
  in-flight snapshot requests are already deduplicated and whether the
  {:children 25 :resources 25} limits are worth raising for network RTT.

Verified with clojure -M:cljs compile app and compile db-worker-node
(0 warnings on both).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/main/frontend/worker/db_worker_node.cljs

@@ -20,6 +20,25 @@
 (defonce ^:private *sse-clients (atom #{}))
 (defonce ^:private *lock-info (atom nil))
 (defonce ^:private *server-list-file (atom nil))
+(defonce ^:private *inflight-by-repo (atom {}))
+(defonce ^:private *max-inflight-by-repo (atom {}))
+
+(defn- invoke-started!
+  "Increments the /v1/invoke in-flight counter for repo and logs a new
+  concurrency high-water mark when observed. Dev-only instrumentation for the
+  server-client migration multi-client baseline (see docs/agent-guide); has no
+  effect on request handling."
+  [repo]
+  (let [n (get (swap! *inflight-by-repo update repo (fnil inc 0)) repo)
+        prev-max (get @*max-inflight-by-repo repo 0)]
+    (when (> n prev-max)
+      (swap! *max-inflight-by-repo assoc repo n)
+      (log/info :db-worker-node-invoke-concurrency-high-water-mark
+                {:repo repo :concurrent n}))))
+
+(defn- invoke-finished!
+  [repo]
+  (swap! *inflight-by-repo update repo (fnil dec 0)))
 
 (defn- server-list-file-path
   [root-dir]
@@ -376,7 +395,9 @@
                       (p/let [_ (when-not (contains? non-repo-methods method-kw)
                                   (let [{:keys [path lock]} @*lock-info]
                                     (db-lock/assert-lock-owner! path lock)))
-                              result (<invoke! proxy method-str method-kw args')]
+                              _ (invoke-started! bound-repo)
+                              result (-> (<invoke! proxy method-str method-kw args')
+                                         (p/finally (fn [] (invoke-finished! bound-repo))))]
                         (send-json! res 200 {:ok true :resultTransit result}))))
                   (p/catch (fn [error]
                              (log-invoke-error! res error method-kw)))))