refactor(head-sync): flatten descendant processing into an iterative drain (leanEthereum#1112)

Rework the gossip head follower so the descendant cascade is a single
iterative loop instead of mutual recursion through three methods.

- Collapse the on_gossip_block return into Store | None. None means the
  block was cached pending an unknown parent; a store means it was
  integrated. This deletes the single-bool HeadSyncResult wrapper and the
  repeated two-channel returns.
- Replace the recursion with a deque worklist over block roots. A long
  backfilled chain can no longer exhaust the stack, and the gossiped block
  is processed explicitly so a failure is a direct early return.
- Delete the reentrancy set. The processed path is now fully synchronous
  (no awaits), so a concurrent entry for the same root cannot interleave;
  the already-in-store check is the sufficient guard.
- Inline the cache-and-backfill helper into its only caller.
- Hash each block once: reuse the gossiped block root and the cached
  child roots rather than recomputing.
- Trim the module, class, field, and inline documentation; correct the
  service write-back comment, which now describes a true invariant.

Tests move from asserting a result wrapper to asserting the store
contract directly, and the reentrancy-guard test is removed with the set.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tcoratger