Done. Both acceptance criteria pass, verified from a fresh clone of the committed tree with forge in --offline mode: forge build compiles cleanly (solc 0.8.30) and forge test passes all 16 tests. The work is committed as 4d6607c containing exactly the two allowed files.

What was delivered

• src/BitPackedMath.sol — a pure library, no storage or external calls. It provides pack/unpack for bit fields described by an offset and width (1–256 bits, must fit in the word), addSat/subSat clamping to the field's range, addChecked/subChecked reverting on overflow and underflow, and mulDiv(x, y, d, Rounding) with explicit Down, Up, and Nearest modes (half rounds up, written as r >= d - r to avoid overflow). Every rejected input has a distinct reason string — zero width, width over 256, field out of word, value too wide, operand too wide, add overflow, sub underflow, division by zero, and mul overflow.

• test/BitPackedMath.t.sol — a test contract importing nothing outside the tree (no forge-std, no vendored code anywhere). Tests assert by reverting via require. Revert paths are exercised through external self-call wrappers with staticcall, comparing the returned revert data byte-for-byte against the expected Error(string) encoding, so each rejection is checked for the exact reason. Coverage includes ordinary values, every field boundary (1-bit fields at bit 0 and 255, a field ending exactly at bit 256, the full 256-bit field, max-value round trips across five widths at both legal extremes), pack overwriting a nonzero field while preserving neighboring bits, saturation exactly at and one past the cap including the 256-bit wraparound case, and the mulDiv rounding edges (exact division, remainder of exactly half, just below and just above half, 1/3, zero operands with an otherwise-overflowing partner, and the largest non-overflowing product).

Two things worth noting: the rules forbid touching foundry.toml, so there is none — forge's defaults handle the standard src//test/ layout, and the wide ^0.8.0 pragma accepts any 0.8.x solc the offline verifier has. And to make sure the suite examines the code rather than blessing it, I ran two mutation checks (removing pack's field-clearing, and flipping the half-rounding comparison); each was caught by the intended test, and the library was restored to its original form before the final verification and commit.