fix(wallet): preserve walletDetails when merkletree-history version is undefined

[zy0n]

The wallet-side merkletree-history-version key lives at [walletPrefix, 'merkleetree_history_version', chainId] — under the same namespace that clearDecryptedBalancesAllTXIDVersions clears via db.clearNamespace(walletPrefix). The engine's
clearUTXOMerkletreeAndLoadedWalletsAllTXIDVersions chain calls into the wallet-clear, wiping the version key but never re-stamping it; the engine's own version key (in ChainSyncInfo namespace) survives.

Result: every cold-sync followed by a refresh saw "wallet version undefined" in loadUTXOMerkletree, treated it as "stale schema, must clear", and wiped the freshly-rebuilt walletDetails — forcing decryptBalances to do a full leaf rescan from creationTreeHeight. A second cold-sync+refresh cycle finally landed the version key in a state where loadUTXOMerkletree skipped the clear path.

Fix: split the two cases the version check was conflating —

• undefined: cold install or post-adjacent-wipe; namespace is empty or already at current schema. Just stamp the version, no clear.
• < CURRENT: real schema migration. Clear and re-stamp.

Schema-bump behavior is preserved: a stored version older than CURRENT still triggers the clear+restamp. fullRedecryptBalancesAllTXIDVersions (driven by treeScannedHeights = [], not the version key) still rescans.

Includes regression test that seeds walletDetails after a clear leaves the version key undefined and asserts loadUTXOMerkletree preserves the seeded heights.