track mnemonic recovery test coverage

[EchoAlice]

Summary

Track follow-up test coverage for the mnemonic recovery loop introduced in #61.

What Tests Already Cover

PR #61 already adds meaningful pure-logic / loop-behavior coverage:

• unit tests for the account-expansion predicate:
  • no accounts
  • below threshold
  • at threshold
  • gap limit is larger than account window (expands)
• loop-simulation coverage for:
  • no expansion
  • one expansion then stop
  • repeated expansion then stop

What Tests Don't Cover

We still need stronger coverage for the real recovery behavior, especially around:

• end-to-end validation that is less slow / noisy / environment-dependent than the current live-chain test
• explicit coverage for final post-recovery balance aggregation across discovered accounts

Follow-up ideas

• add table-driven cases for needs_account_expansion(...) if we want more compact/maintainable predicate coverage
• add explicit post-convergence balance aggregation assertions
• add mixed Sapling/Orchard highest_active cases after Orchard support lands
• add deterministic recovery-loop test fixtures to exercise end-to-end logic without requiring a live network. Including just-below, exact-boundary, and just-above threshold cases.