Permissionless DELETE on expiry

[arrivets]

Refer to https://github.com/Arkiv-Network/arkiv-architecture/blob/main/arkiv/tokenomics/arkiv-fee-spec.md

Spec §2: DELETE_ENTITY must be callable by any account once expiresAt has
passed. Today's EntityRegistry / precompile presumably restricts DELETE to the
owner.

Changes:

• contracts/src/EntityRegistry.sol and crates/arkiv-node/src/precompile.rs:
  widen the authorization check on DELETE_ENTITY to:
  caller == owner || block.timestamp >= expiresAt
• Tests: owner-deletes-before-expiry (allowed), stranger-deletes-before-expiry
  (rejected), stranger-deletes-after-expiry (allowed), owner-deletes-after-expiry
  (allowed).

Note: this is a behavior change to the registry independent of fees — could
land before #86/#87. Concentrated-expiry safety argument in §2 relies on this.