fix: Reject duplicate BitName registrations within a single block

[giaki3003]

What's wrong

State::validate_filled_transaction (lib/state/mod.rs) checks that a registered name is not already taken, but it reads the pre-block DB, so it cannot see a registration earlier in the same block. block::validate / block::prevalidate (lib/state/block.rs) reject double-spent outpoints but do not look at registration name hashes at all.

Two registrations of the same name can therefore both be individually valid — distinct nonces give distinct commitments, so each burns a real reservation — and both connect. The second apply_registration overwrites the canonical bitnames row, and that overwrite is not invertible: on disconnect the second revert_registration deletes the shared row, and the first then fails with Error::Missing. A node that connected such a block can no longer disconnect it, so it is stuck on that branch across a reorg.

The fix

Add a per-block uniqueness check over registration_name_hash() in both validate and prevalidate, right next to the existing double-spend check, returning a new Error::DuplicateBitNameRegistration. As defence in depth, apply_registration now returns BitName::AlreadyRegistered instead of overwriting an existing row. Happy to move the new variant under error::BitName if you prefer it there.

Tests

Adds validate_rejects_duplicate_registration_within_block: two registrations for one name, each asserted to pass validate_filled_transaction, with the containing block asserted to be rejected.

Finding report (access-controlled): https://giaki3003.tech/#/findings/20260610-2220-rustl2-bitnames-bitassets-same-block-duplicate-registration-disconnect-noninvertible

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First of a short series of 1 fixes for this repo, based on master. The rest build on this branch and will follow.