Committed on main as 8ba40d1. Everything is in place and verified.

What was built

• src/BatchNFT.sol: ERC721A-style ERC-721. Ids start at 1, mint(quantity) is public and free for 1 to 20 tokens up to a 10,000 cap. One packed slot (owner plus uint64 timestamp) is written per batch. ownerOf walks back at most 19 slots. A transfer writes the moved token's slot and back-fills the next id with the previous slot data when that id is minted and still empty. Full approvals, both safeTransferFrom overloads with the receiver check, ERC-165, tokenURI as base string plus id, no burn, no enumerable extension.
• src/BaselineNFT.sol: OpenZeppelin ERC721 with the same mint rules, one _mint per id.
• lib/: OpenZeppelin Contracts v5.1.0 subset and forge-std v1.9.4 vendored as plain files. No submodules, so the offline verifier can build.
• README.md: design, storage model, parity decisions with OpenZeppelin, gas table, assumptions, deployment parameters, and operational responsibilities.

Checks run

Tests cover mint boundaries and failure cases, one Transfer event per id, single slot written per batch, first, middle and last transfers with raw-slot checks, no overwrite of a following batch's first slot, approvals and operators, every safeTransferFrom failure mode, and a differential fuzz that applies identical sequences to both contracts and compares ownerOf, balanceOf, getApproved, isApprovedForAll, and success versus revert per call. A mutation check confirmed the differential test fails immediately when the back-fill is broken.

Gas for mint(20) is roughly 94k on BatchNFT against 531k on the baseline. The first transfer out of a batch costs more on BatchNFT, which the README documents as the expected trade-off.

Noted assumptions: minting is unrestricted with no owner role, base URI is immutable after deployment, and the contracts target the Cancun EVM. No deploy script was requested and none was written.

Committed. The work is complete and verified.

Outcome. The gas and size report is at artifacts/gas.md, the snapshot is committed as .gas-snapshot, and the full suite passes. I found no size defect and no implementation defect, so no findings file was written.

Sizes. No contract exceeds the 24,576-byte deploy limit.

What I added. A new test file, test/GasSnapshot.t.sol, with one isolated test per public function for each contract from an identical fixture, so every snapshot line names the function it measures. It also has a logging test that measures the assignment's scenarios as single calls with state reverted between them. The existing tests were left as they were.

Headline numbers, isolated calls.

Where BatchNFT costs more. Every first transfer out of a batch, at 1.5x to 2.4x the baseline, because it pays the ownership walk, a fresh slot write for the moved id, and a back-fill write for the next id. Any owner lookup on an unwritten id, up to 6.8x at 19 steps, and approve inherits the same walk. Second transfers of a token cost about 4,000 more. The runtime is 111 bytes larger. Minting 5 and 20 tokens is 57 and 82 percent cheaper, and minting 1 is a wash.

Verification. forge build is clean, all 115 tests pass, and forge snapshot --check passes on repeated full-suite runs. The fuzz lines in the snapshot only shift when a filtered subset is run, which the report notes. The artifacts/ directory was hidden by a repo-local git exclude rule, so I force-added the report to make sure it is in the commit.