chore(circuits): Add some comments for rollup circuits (#17837)

Add some comments

Author: sirasistant

noir-projects/noir-protocol-circuits/crates/rollup-lib/src/abis/root_rollup_public_inputs.nr

@@ -5,7 +5,12 @@ use types::{
 };
 
 pub struct RootRollupPublicInputs {
-    // Snapshot of archive tree before/after this rollup has been processed
+    // Root of archive tree before/after this rollup has been processed
+    // We don't submit the nextAvailableLeafIndex to L1, it's validated by the circuits themselves:
+    // During an epoch, we assert that the previous archive is the new archive from the next rollup merged in.
+    // However, to validate the very first previous archive of an epoch, the validation is more succinct:
+    //  - If you try to set a higher nextAvailableLeafIndex (skip), you won't be able to provide a valid preimage to `validate_previous_block_header_in_archive`
+    //  - If you try to set a lower nextAvailableLeafIndex (overwrite), you'll fail the zero leaf check in the tree insertion code
     pub previous_archive_root: Field,
     pub new_archive_root: Field,
     pub checkpoint_header_hashes: [Field; AZTEC_MAX_EPOCH_DURATION],

noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_merge/utils/merge_checkpoint_rollups.nr

@@ -18,9 +18,7 @@ pub fn merge_checkpoint_rollups(
         right.checkpoint_header_hashes,
     );
 
-    // TODO(Miranda): combine fees with same recipient depending on rollup structure
-    // Assuming that the final rollup tree (block root -> block merge -> root) has max 32 leaves (TODO: constrain in root), then
-    // in the worst case, we would be checking the left 16 values (left_len = 16) against the right 16 (right_len = 16).
+    // We can't merge fees for the same recipient since the l1 contract iterates per checkpoint.
     let fees = array_merge(left.fees, right.fees);
 
     CheckpointRollupPublicInputs {

noir-projects/noir-protocol-circuits/crates/types/src/abis/checkpoint_header.nr

@@ -16,6 +16,8 @@ pub struct CheckpointHeader {
     // The timestamp of the slot. All blocks in the checkpoint must have a timestamp less than or equal to this value.
     pub timestamp: u64,
     pub coinbase: EthAddress,
+    // This field is an optional arbitrary l2 address set by the block builder. It's unchecked, and allows
+    // off chain functionality between builders and tx submitters.
     pub fee_recipient: AztecAddress,
     pub gas_fees: GasFees,
     pub total_mana_used: Field,