Add deterministic engine-scheduler tests

The fake plugin reports each makeCurrencyEngine call through an
onEngineCreate hook, so tests can observe creation order. Cases cover
concurrency-limited draining, waitForCurrencyWallet bumping a queued
wallet to the front, cold wallets bypassing the queue, a deleted queued
wallet giving up its place, and balanceMap identity across unchanged
balance reports.

Author: j0ntz

CHANGELOG.md

@@ -3,8 +3,10 @@
 ## Unreleased
 
 - added: Per-wallet `walletCache.json` with each wallet's name, fiat code, enabled token IDs, and last-known balances. Currency wallets now emit their API objects as soon as this cache loads, before their engines exist, so the GUI can render the wallet list immediately at login. Engine-backed methods wait for the engine internally and reject if it fails or the wallet is deleted. First login (no cache) behaves exactly as before.
+- added: Cached wallets' engine startup is staggered through a limited-concurrency queue (8 at a time) instead of all racing at login. Asking for a wallet via `waitForCurrencyWallet`, calling an engine- or storage-backed method, or un-pausing it moves it to the front of the queue. Wallets without a cache skip the queue, so first login is unaffected.
 - changed: `waitForCurrencyWallet` and `waitForAllWallets` now resolve when the wallet object exists, which can be before its engine loads.
 - changed: `wallet.otherMethods` is `{}` until the wallet's engine loads, then switches to the engine's methods.
+- changed: `EdgeCurrencyWallet.balanceMap` keeps its object identity when an engine re-reports an unchanged balance.
 
 ## 2.47.1 (2026-07-17)