chore: resolve stale TODOs in epoch processing (#285)

Resolves two stale TODOs in epoch processing:

1. **`process_pending_deposits.zig`**: Removed false-alarm TODO
questioning whether `withdrawal_credentials` matches the spec — it does.
`PendingDeposit.withdrawal_credentials` is `Bytes32`, exactly matching
the consensus spec.

2. **`process_randao_mixes_reset.zig`**: Replaced mystery `TODO inspect
why this clone was needed` with an explanation. The clone is necessary
because `set()` at a different gindex may CoW-rebranch the persistent
merkle tree, invalidating the node reference returned by `get()`.

🤖 Generated with AI assistance

Co-authored-by: lodekeeper-z <lodekeeper-z@users.noreply.github.com>

Author: lodekeeper-z

src/state_transition/epoch/process_pending_deposits.zig

@@ -129,7 +129,7 @@ fn applyPendingDeposit(
 ) !void {
     const validator_index = epoch_cache.getValidatorIndex(&deposit.pubkey) orelse null;
     const pubkey = &deposit.pubkey;
-    // TODO: is this withdrawal_credential(s) the same to spec?
+
     const withdrawal_credentials = &deposit.withdrawal_credentials;
     const amount = deposit.amount;
     const signature = deposit.signature;

src/state_transition/epoch/process_randao_mixes_reset.zig

@@ -15,7 +15,8 @@ pub fn processRandaoMixesReset(
     var old = try randao_mixes.get(current_epoch % preset.EPOCHS_PER_HISTORICAL_VECTOR);
     try randao_mixes.set(
         next_epoch % preset.EPOCHS_PER_HISTORICAL_VECTOR,
-        // TODO inspect why this clone was needed
+        // Clone needed: set() at a different gindex may CoW-rebranch the
+        // tree, invalidating the node reference returned by get().
         try old.clone(.{}),
     );
 }