fix: complete wallet sync through the late activity gate

[ovitrif]

Description

• Pipelines Electrum wallet scan requests while keeping the selected transaction hidden until explicit release. The readiness probe now checks 500 requests so the RN restore setup can detect a sync timeout before restoring.
• The review stack's test environment runs this change; the test run that needs it is reported in a comment here.
• Checked: The original serialized implementation failed the latency regression; the revised fixture passed all 14 socket and helper tests, including both batch wire formats and release during an in-flight scan. A live staging smoke using an existing unspent receive completed 500 responses directly in 0.140 seconds, through the held gate in 0.260 seconds and through the released gate in 0.269 seconds. Held activity stayed hidden, released activity matched upstream, and the raw transaction was unchanged. This proves the fixture on the coordinator; application restore and migration behavior remain for the

Out of Scope

• None.

[ovitrif]

Closing: an Electrum sync gate our review stack needed for synonymdev/bitkit-ios#852, which has merged; no test run used it.