[类型 / Types] 按需评估 JSON scalar 与 open record / evaluate JSON scalars and open records on demand

中文说明

评估显式 JSON scalar 或 open record 作为 typed map 之外的逃生通道。该能力会引入新的 codec、wire、比较、索引、migration 和非穷尽 match 语义；只有真实外部 payload 无法用 map text T 或命名 ADT 表达时再推进。

进入条件：明确调用方、至少一个不可用 typed map 解决的样本、大小/深度预算，以及是否需要字段级查询或索引。当前无 milestone，P2 deferred。

English Description

Evaluate an explicit JSON scalar or open record as an escape hatch beyond typed maps. This adds codec, wire, comparison, indexing, migration, and non-exhaustive-match semantics. Proceed only when a real external payload cannot be represented by map text T or named ADTs.

Entry requires a concrete caller, a sample that typed maps cannot model, size/depth budgets, and a decision on field-level query/index needs. No milestone; P2 deferred.

No files, tests, or entry points are named. Start by identifying a concrete caller and collecting an external payload that cannot be represented by map text T or a named ADT. Done means documenting the sample, size and depth budgets, and the decision on field-level querying or indexing before implementation proceeds.