feat(kits): add si kit

[the-simian]

SI base + derived. Pair with the imperial kit issue.

Units: meter, kilogram, second, kelvin, mole, ampere, candela.
Derived: square meter, cubic meter, newton, joule, watt, pascal, hertz.

Scoping question (decide before scaffolding): the shipped geometry kit already covers metric length/area/volume + imperial. Three options:

1. Retire geometry; ship full si + full imperial. Demo migrates.
2. Keep geometry as a curated "common shapes" subset; add fuller si + imperial alongside.
3. Rename geometry to a domain kit (not a unit-system kit) and decouple it.

[the-simian]

PR #38 addresses this issue differently than originally scoped. The foundational refactor establishes one foundational kit per dimension (length, volume, mass, temperature), each canon-cited against NIST / BIPM / ISO / IAU standards. SI atoms ship in their dimension's foundational kit alongside customary atoms; there is no single kits/si separate from kits/imperial.

The scoping question raised in this issue ("retire geometry vs. keep alongside vs. rename to a domain kit") resolved as: geometry stays as a composition kit that re-exports LENGTH + VOLUME atoms from the new foundational kits and keeps its 40+ shape derivations.

Closing in favor of the new shape is your call. Leaving open in case the original "shipped SI base + derived in one kit" framing still has consumer demand.