bug: spark exit funding is not watched after a restore #1296

Twin: synonymdev/bitkit-android#1877
Refs: #1217

Problem

After the wallet is restored from its recovery phrase on a new device, exit funding sent to a funding address at index 1 or above isn't watched. A user who restores after a failed salvage can leave that funding unrecovered, with no automatic way back.

Why

• The next funding index is a counter in UserDefaults.standard, under sparkExitNextFundingIndex-<network>-<walletIndex> (Bitkit/Managers/SparkExitManager.swift:1174-1179, set at :964, given back at :545-546).
• The only other input is the exit record stored on the device (SparkExitMath.nextFundingIndex, Bitkit/Services/SparkExit/SparkExitFunding.swift:585-588).
• Bitkit's settings backup doesn't include the counter (Bitkit/Models/SettingsBackupConfig.swift).
• The Spark exit backup holds only each leaf with its ancestor chain, with no funding data (unilateral_exit_backup.rs#L29-L34 at Spark SDK 0.28.0).
• So after a restore from the phrase, the counter reads 0 and no record exists.
• Unlike Android, an iCloud or Finder device backup restored onto a new iPhone can bring UserDefaults back. A restore from the phrase alone can't.

Raised by @pwltr in review 5850248225 on the Android twin, and checked against the SDK source.

Fix options

• Scan funding addresses 0..N at start, up to a gap limit.
• Or back up the counter with the exit backup or the settings backup.

Test

Set the counter to 3, clear UserDefaults and the exit record, restore from the seed, and assert that the next exit finds a UTXO at index 2.