Bump zcash_voting to 0.5.8 — bundle identity persistence, HTTP runtime drop fix

[greg0x]

Why

Two release-blocking issues for the upcoming governance flow:

• Bundle note identities can drift after setup. setup_bundles only recorded
  note positions, so if the same position later resolved to a different note,
  governance PCZT construction would silently sign over substituted inputs.
  Persisting full identity hashes and rejecting same-position substitutions
  closes that window.

• HTTP transport panicked when dropped under Tokio. The tree-sync feature
  builds a multi-thread Tokio runtime inside HyperTransport. Dropping it
  from inside an active Tokio context (which happens during normal teardown
  on the wallet side) trips Tokio's "drop runtime in async context" panic.
  Wrapping it in a BlockingRuntime that calls shutdown_background on drop
  avoids the panic without changing call-site behavior.

Migration

Adds DB schema v8: new bundles.note_identity_hashes_blob column. Existing
rows are preserved with NULL identity hashes; the column is also part of
001_init.sql so fresh databases get it directly. Migration is idempotent
(guards on column_exists).

Tests

• cargo test -p zcash_voting storage:: — covers the v7→v8 migration path
  and identity-hash persistence/readback.
• cargo test -p zcash_voting --features client-tree-sync blocking_runtime_drop_does_not_panic_inside_tokio_context
  — regression test for the runtime drop panic.