Composite: don't undo writes made while a full sync is scanning

A write made during a full sync could be scanned in the replica but not in
the (read-ahead) source scan, so the diff removed it, and echo tracking then
consumed the server's echo. Keys written during a sync are now left out of
the diff. Deterministic regression test with a read-ahead source.

Also exclude -0 from the Extended JSON property test (JSON has no -0).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Qxbkp4tA7G5SCWjzWuxmE

DESIGN.md

@@ -69,7 +69,7 @@ emit a `change` event after each write, so every store emits change events.
 | `applyRemove(keys)` → public `removeMany`              |                                                                                                      |
 | `applyWriteIfAbsent(entries)` → public `writeIfAbsent` | `true` = written; atomic per key (needed by events)                                                  |
 
-**Values** are JSON plus `bigint` and `Uint8Array`. Anything crossing the network is encoded as
+**Values** are JSON plus `bigint` and `Uint8Array` (as in JSON, `-0` becomes `0` and `undefined` properties are dropped). Anything crossing the network is encoded as
 Extended JSON (`<bigint: 123n>`, `<uint8array: abcd>`); IndexedDB stores them natively.
 
 **Events** (`EventEmitter`, shared by all nodes of a storage): `change` →