refactor: replace core.async with promesa for all async operations

- Store protocol now returns promesa promises instead of core.async channels
- MemoryStore: `(p/resolved val)` replaces chan+put!+close! ceremony
- IDBStore: `p/create` with resolve/reject wraps IDB callbacks
- sync.cljc: CLJ uses `p/vthread`, CLJS returns native Promise chains
- core.cljc: `p/let` replaces go blocks, timer-based sync loop replaces
  go-loop+alts!, debounced push replaces kick channel
- Tests use `deref` with timeout on promesa promises
- Todomvc example uses `p/let` instead of go/<!

src/pocketbook/store.cljc

@@ -1,23 +1,23 @@
 (ns pocketbook.store
   "Storage protocol for Pocketbook.
-   All methods return core.async channels.")
+   All methods return promesa promises.")
 
 (defprotocol PStore
   (put-doc! [store doc]
     "Write a document to the store. doc is a map:
        {:id str, :value any, :version int, :updated int, :deleted bool, :synced bool}
-     Returns a channel that closes on success.")
+     Returns a promise that resolves on success.")
 
   (docs-by-prefix [store prefix]
     "Get all documents whose id starts with prefix.
-     Returns a channel yielding a vector of doc maps.")
+     Returns a promise yielding a vector of doc maps.")
 
   (get-meta [store key]
     "Get a metadata value by key.
-     Returns a channel yielding the value, or nil if not found.")
+     Returns a promise yielding the value, or nil if not found.")
 
   (set-meta! [store key value]
-    "Set a metadata value. Returns a channel that closes on success.")
+    "Set a metadata value. Returns a promise that resolves on success.")
 
   (close-store! [store]
     "Close the store and release resources."))