feat(kits): add imperial kit

[the-simian]

Imperial roster; pair with the si kit issue.

Units: inch, foot, yard, mile, pound, ounce, fluid ounce, gallon, quart, pint, cup.

Stress-tests mixed-system cross-dim forging (an SI-defined recipe converting against imperial inputs). Same scoping question as the si issue re: overlap with the already-shipped geometry kit.

[the-simian]

Same disposition as #3: PR #38 distributes imperial / customary atoms (inch, foot, yard, mile, pound, ounce, fluid ounce, gallon, quart, pint, cup) across the foundational kits/length, kits/volume, and kits/mass rather than shipping a single kits/imperial kit. All US/UK splits are now explicit (US vs UK fl oz, US vs UK pint/quart/gallon, Customary vs Legal FDA cup, etc.).

Mixed-system cross-dim forging works against the foundational atoms today (a recipe defined in SI converts cleanly to imperial-input cups because both live in the same VOLUME dimension).

Closing in favor of the new shape is your call.