Pure UTC date primitives shared by date components (x-date-picker,
x-calendar). No DOM, no side effects — every function is a value
transform. Dates are JS `Date` objects pinned to UTC midnight; ISO
strings are `"YYYY-MM-DD"`.

`first-day-of-week` (`fdow`) is `0`=Sunday .. `6`=Saturday. The
`start-of-week` / `end-of-week` / `month-grid` functions default to
`0` (Sunday) so existing single-arity callers keep their behaviour.

Shared form-control policy for form-associated components: ElementInternals
validity + form value, and the inline error-display recipe.

The two decisions here — constraint `validity` and the `error-describedby`
token — are pure functions, separated from their effects (`set-validity!` /
`sync!` / `apply-error-display!`), so the policy that maps a component's
`error` / `required` / emptiness onto native validity and invalid markers
lives in one place and is unit-testable — the browser test harness cannot
exercise the ElementInternals / DOM effects, but it can test the projections.

A component supplies only its genuine per-control variation — what counts as
*empty*, which element is the validity *anchor*, its *value* projection, any
non-default *missing message*, and (for describedby) any author-supplied id —
and composes this policy.

Shared overlay root management for portal-based components.
Provides document-level fixed layers that escape CSS stacking contexts.