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/db/subs_loader.cljs

@@ -8,6 +8,22 @@
 
 (defonce ^:private *batch (atom {}))
 
+(defonce ^:private *stats (atom {:dedup-hits 0 :new-entries 0 :flushes 0 :batch-sizes {}}))
+
+(defn get-stats
+  "Snapshot of subs-loader batching counters: :dedup-hits counts load! calls
+  that reused an already-pending slot request within the same unflushed
+  batch window (see `load!`); :batch-sizes tallies entries per slot kind
+  across flushes. Dev-only instrumentation for the server-client migration
+  latency baseline (see docs/agent-guide); does not affect batching or the
+  `limits` above."
+  []
+  @*stats)
+
+(defn reset-stats!
+  []
+  (reset! *stats {:dedup-hits 0 :new-entries 0 :flushes 0 :batch-sizes {}}))
+
 (defn- reject!
   [entries error]
   (doseq [{:keys [result]} entries]
@@ -45,6 +61,13 @@
   []
   (when @state/db-worker-ready?
     (let [batch (take-batch!)]
+      (swap! *stats
+             (fn [s]
+               (-> s
+                   (update :flushes inc)
+                   (update :batch-sizes
+                           #(merge-with + %
+                                        (frequencies (map (comp slot-kind first :slot-key) batch)))))))
       (doseq [[graph-id graph-entries] (group-by :graph-id batch)
               entries (request-groups graph-entries)]
         (-> (state/<invoke-db-worker :thread-api/get-render-snapshots
@@ -72,9 +95,11 @@
   [graph-id slot-key schedule!]
   (let [entry-key [graph-id slot-key]]
     (if-let [result (get-in @*batch [entry-key :result])]
-      result
+      (do (swap! *stats update :dedup-hits inc)
+          result)
       (let [result (p/deferred)
             schedule? (empty? @*batch)]
+        (swap! *stats update :new-entries inc)
         (swap! *batch assoc entry-key
                {:graph-id graph-id :slot-key slot-key :result result})
         (when schedule?

src/main/frontend/state.cljs

@@ -63,6 +63,28 @@
                  (reset! ready? (fn? worker))))
     ready?))
 
+(defonce ^:private *invoke-stats (atom {}))
+
+(defn get-invoke-stats
+  "Snapshot of db-worker invoke counters, keyed by thread-api method kw. Each
+  entry is {:count N :total-ms N :max-ms N}. Dev-only instrumentation for the
+  server-client migration latency baseline (see docs/agent-guide); does not
+  affect invocation behavior."
+  []
+  @*invoke-stats)
+
+(defn reset-invoke-stats!
+  []
+  (reset! *invoke-stats {}))
+
+(defn- record-invoke-stat!
+  [qkw elapsed-ms]
+  (swap! *invoke-stats update qkw
+         (fn [{:keys [count total-ms max-ms] :or {count 0 total-ms 0 max-ms 0}}]
+           {:count (inc count)
+            :total-ms (+ total-ms elapsed-ms)
+            :max-ms (max max-ms elapsed-ms)})))
+
 (defn <invoke-db-worker
   "invoke db-worker thread api"
   [qkw & args]
@@ -70,11 +92,13 @@
     (when (nil? worker)
       (prn :<invoke-db-worker-error qkw)
       (throw (ex-info "db-worker has not been initialized" {})))
-    (p/let [result (apply worker qkw args)]
-      (if (or (instance? ExceptionInfo result)
-              (instance? js/Error result))
-        (p/rejected result)
-        result))))
+    (let [started-at (js/performance.now)]
+      (-> (p/let [result (apply worker qkw args)]
+            (if (or (instance? ExceptionInfo result)
+                    (instance? js/Error result))
+              (p/rejected result)
+              result))
+          (p/finally (fn [] (record-invoke-stat! qkw (- (js/performance.now) started-at))))))))
 
 (def ^:private export-block-text-indent-styles #{"dashes" "spaces" "no-indent"})
 

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)))))