feat(validation): type inference for validators and schema builders

[halvardssm]

Summary

• Add Zod-inspired type inference for the validation namespace.
• New validation/infer.ts with InferInput/InferOutput plus composition helpers
  (InferMemberOutput, InferObjectOutput, InferCombinationOutput) that read
  the schema's ~standard.types field.
• The schema() builder now sets ~standard.types, so the inferred input/output
  types resolve to the schema's Input/Output (the generic params alone cannot
  infer Input because it is structurally absent from StandardSchemaV1).
• array, object, and combination builders are now generic over their
  options, so composed types are inferred:
  • array({ items: string() }) -> string[]
  • object({ properties }) -> object shape (all properties optional, since
    JSON Schema's required is string[] and widens array literals)
  • combination({ anyOf }) -> union of member output types
• validate/validateAsync/parse/parseAsync accept unknown input
  (matching Zod's parse(data: unknown): Output) and return
  Result<InferOutput<S>> / InferOutput<S> instead of unknown.
• Re-export InferInput/InferOutput (and helpers) from the package entrypoint.
• Add validation/infer.test.ts with compile-time IsExact assertions for
  scalars, array, object, combination, and the parse/validate signatures.
• Document type inference in validation/README.md.

Verification

• Type-checked all validation/ source + test files with tsc against the real
  @standard-schema/spec type definitions (0 errors).
• Ran the existing runtime behavior through Node (--experimental-strip-types):
  validator.test.ts (12/12), utils.test.ts (14/14), and a broad
  json_schema.test.ts subset covering scalars, formats, arrays (prefixItems,
  contains, uniqueItems), nested objects (additionalProperties, patternProperties,
  minProperties), and combinations (allOf/anyOf/oneOf/not) — all pass with
  behavior unchanged. New infer.test.ts runtime sanity passes and its
  compile-time assertions hold.

Note: deno task check / deno task test were not run in this sandbox
(Deno is not installed here); CI will run them. The added ~standard.types
field uses undefined runtime values as a type-level carrier (per the
Standard Schema spec), and existing assertObjectMatch schema-shape checks
use partial matching, so they remain green.