[Feat]: Expose when a node contenthash last changed

Context

Browse ranks published apps and wants a freshness signal: how recently an app content was updated. The natural source is the dotNS content resolver, since a redeploy updates the label contenthash.

Current behavior

• contenthash(bytes32 node) returns only the current blob, with no timestamp and no history.
• DotnsContentResolver emits ContentHashUpdated(bytes32 indexed node, bytes hash) on every write (contracts/resolvers/IDotnsContentResolver.sol), but the event carries no timestamp either.
• So the only way to answer "when did this node's contenthash last change" is to scan ContentHashUpdated logs for the node, take the latest, and read that block's timestamp: an event/log scan plus a block-to-timestamp lookup.

Ask

Make the last-updated time cheaply readable from a view. Store an updatedAt (uint64 block timestamp) per node and expose a view such as contentHashUpdatedAt(bytes32 node) returns (uint64). One extra SSTORE per write, O(1) read, no log infrastructure needed. Could also be returned alongside the hash.