Done. Both acceptance criteria pass from a cold cache: forge build succeeds and forge test reports 21 passed, 0 failed.

What's in the diff — exactly two files, nothing else:

• src/Bits.sol — a pure library Bits with seven helpers: get/set/clear (bounds-checked, revert IndexOutOfBounds), popCount, mostSignificantBit (reverts ZeroHasNoSetBits), isPowerOfTwo, rotateLeft. No storage, no external calls.
• test/Bits.t.sol — 21 discoverable tests (16 concrete + 5 fuzz) importing only ../src/Bits.sol. Assertions revert with a message; revert paths are checked by staticcalling external wrappers and comparing the full returndata against the expected custom-error encoding, since internal library calls are inlined and can't be caught.

One deviation worth flagging. The task says to write the build configuration by hand, but your rules restrict me to those two paths and auto-reject anything touching foundry.toml. The rules win, and they turn out to cost nothing: the directory layout is the configuration here. Foundry's defaults are src/test and it resolves the project root from .git, so the tree builds with no config file at all. That also removes a failure mode — there's no config to disagree with the verifier's environment.

Two notes on offline reproducibility:

• Nothing is vendored, no submodules, no lib/. I verified with forge build --offline / forge test --offline after deleting out/ and cache/.
• The one thing I can't control is the verifier's solc. There is no foundry.toml to pin a version, so pragma solidity ^0.8.4 is deliberately wide — forge's auto-detect prefers an already-installed compiler matching the range, so any cached 0.8.x ≥ 0.8.4 works without a download. (My machine had none cached and fetched 0.8.30 once; a verifier with no solc and no network would fail on any Foundry project regardless of what I wrote.)

On the suite being non-vacuous. Expected values come from naive loops in the test file (refPopCount, refMostSignificantBit, refRotateLeft) written from the documented behavior, not from the library's bit tricks. To confirm the tests actually bite, I ran mutation testing in a throwaway copy outside the repo: 8 seeded bugs (byte-lane popcount that wraps at max, off-by-one bound, dropped msb step, msb returning 0 instead of reverting, isPowerOfTwo(0) true, unreduced rotate amount, set-as-toggle, wrong clear mask) — all 8 were caught, baseline clean.

Two things that check the library rather than my implementation: popCount(type(uint256).max) returns 256, where the textbook byte-lane multiply silently returns 0 — that's why the library widens to 32-bit lanes before the final sum. And my first draft of a fuzz assertion was wrong (word >> index drops low bits, not high ones); it failed, and I fixed the assertion, not the library.

The two files are committed (2f45e41); build artifacts are removed, so the tree holds only src/ and test/. I did not create test/scratch/ — the mutation runs went to /tmp to avoid test/scratch/** being compiled into my own verification runs.