Schema helpers for D-core.

This module is intentionally small:
- Applies strictness (:strict/:lax) to Malli map schemas (:closed true/false)
- Validates values and throws typed, non-retriable exceptions for poison handling
- Provides temporal schema constructors (`temporal`) that make the expected
  date/time domain *and* wire dialect an explicit, declarative per-field choice
  (roadmap 02, B6).

The temporal contract (roadmap 02, B1).

Two vocabularies, deliberately separate:

- **Domains** are the *internal* representation. Everything internal is UTC:
  a point in time is a `java.time.Instant`, a span is a `java.time.Duration`,
  and `:local-date` is a civil date that is deliberately *not* a point in time.
  Zone-bearing and wall-clock types (`:local-date-time`, `:zoned-date-time`,
  `:offset-date-time`) are **ingress formats only** — they are resolved to an
  `Instant` at the boundary and never travel internally (D10).

- **Dialects** are the *encodings* used at a boundary. A dialect is a named
  value, chosen explicitly, never inferred (D6). `:epoch-ms` is the platform
  default (D3b) and is narrow: it can only carry `:instant`, at millisecond
  resolution.

Nothing here guesses. A number without a dialect is an error, a zone-less
datetime is an error (D11), and a value a dialect cannot represent is an error
rather than a silent narrowing (D5).