fix: import ProposedHeaderLib in Rollup.t.sol

The test calls ProposedHeaderLib.hash at lines 913/915 (added in A-1373) but
only imported the ProposedHeader struct, so a from-scratch forge build fails
with an undeclared-identifier error. Latent until now because CI restored a
cached l1-contracts build; a cache miss exposed it.

Author: spalladino

l1-contracts/test/Rollup.t.sol

@@ -12,7 +12,7 @@ import {Registry} from "@aztec/governance/Registry.sol";
 import {Inbox} from "@aztec/core/messagebridge/Inbox.sol";
 import {Outbox} from "@aztec/core/messagebridge/Outbox.sol";
 import {Errors} from "@aztec/core/libraries/Errors.sol";
-import {ProposedHeader} from "@aztec/core/libraries/rollup/ProposedHeaderLib.sol";
+import {ProposedHeader, ProposedHeaderLib} from "@aztec/core/libraries/rollup/ProposedHeaderLib.sol";
 
 import {
   IRollupCore,