Done. The project builds, tests, and passes the format check fully offline.

What was delivered (five files, no dependencies, no submodules, no forge-std):

• foundry.toml pins solc 0.8.26, which was already in the local compiler cache before this session, with ffi off, no filesystem permissions, and no remappings.
• src/MerkleProofLib.sol is the only contract: a pure internal library with verify and computeRoot. Hashing is order-dependent, and the leaf's index bits decide which side each sibling is on. It declares three custom errors: ProofTooLong, IndexOutOfRange, and RootMismatch.
• test/MerkleProofLib.t.sol holds the naive oracle, a thin harness so reverts can be caught externally, and the suite. The oracle pads leaves to a power of two, builds every layer in memory, and reads proofs straight off the layers. The cheatcode interface is declared inline.
• README.md names forge test as the single command, records the expected result of 12 passing tests, and documents the tree convention, the errors, the second-preimage caveat for callers, the zero-padding assumption, and that there is nothing to deploy.
• .gitignore excludes build output.

Test coverage across trees of 1 to 64 leaves at every position: every builder proof verifies and both sides agree on the root, repeated and fully identical leaves, sibling order flipped at a fuzzed level (accepted only when the oracle shows equal children), proofs one element too short and too long, right proof against a wrong root, leaves the tree never held, and out-of-range indices. Refusal tests assert the exact error selector and arguments, not just a revert. Fixed tests cover the single-leaf tree, a two-leaf tree with the order flipped, a 257-element proof, and the 256-level maximum.

Verification results: clean build with no warnings, 12 of 12 tests passing, and forge fmt --check clean. The files are left uncommitted in the working tree since committing was not requested.

One caveat to note: the offline build relies on solc 0.8.26 being present in the verifier's local svm cache. I chose it because it was the oldest cached version on this machine, which makes it the most likely one in the base image.