spec(field): refuse a homeAmount rateField whose declared scale is below 12, and document CURRENCY_FRACTION_DIGITS as CLDR fraction digits, not ISO 4217 minor units (multi-currency, epic #8345) #30103

epic:#8345#1 · os-bill · session_01Utp6CZWjnSeed8H4dMjt9d

Part of #8345. Filed by the epic PM into the domain:spec seat's queue (spec children of the epic go to the spec seat; seat-lifecycle 〈Epic 子树车道〉). Owed by the predecessor's sign-off ledger (brief 6540325320). ⛔ Not a claim.

Ruling-ref: 6469382727

维护者速读

• 改了什么： 两件 spec 事：① homeAmount.rateField 指向的数字字段若声明了小于 12 的 scale，建模时就拒收（裁决 6469382727 定了汇率 scale 12）；② 写明 CURRENCY_FRACTION_DIGITS 是 CLDR 小数位，不是 ISO 4217 最小单位，并列出两者不同的币种。
• 为什么改： 引擎存汇率全精度，声明 scale 4 只影响显示，作者看到的汇率与实际折算用的不一致；文档写 ISO 4217，实际查的是 CLDR，约 17 个币种（如 IQD、HUF、COP、IDR、PKR）小数位不同。
• 风险与回滚： ① 收窄了 18.3.0 已发布的接受集，已声明 scale<12 的应用会在校验时报错，changeset 须写收窄说明；② 纯文档。
• 先看： PR feat(objectql): store a record-mode amount converted into the home currency on every write (multi-currency M2-2) #28302 正文「Hand-off points」第二条；评审 6539117549 S2；objectql: write-time home projection — rate lookup by document date, decimal arithmetic, minor-unit rounding, missing rate refuses, computed fields read-only (multi-currency M2-2, epic #8345) #27192 open question 3。
• 两分钟验证： 声明 rateField 为 scale: 4 的对象校验被拒，错误点名字段与下限 12；scale: 12 通过。
• 需要您做的： 无（收窄须一次契约级复审）。

Task

• In the object-level judge of homeAmount pointers (packages/spec/src/data/object.zod.ts#refuseDishonestHomeAmount), refuse a rateField whose declared scale is below 12; the message names the field and the floor. Decide and state what an undeclared scale means.
• TSDoc on CURRENCY_FRACTION_DIGITS and content/docs/data-modeling/currency.mdx: CLDR fraction digits; list each code where CLDR differs from ISO 4217 minor units, measured from the table, not recalled.
• Tests: scale 4 refused, 12 and 14 accepted, undeclared per the decision.

Lane: spec · Size: S · Review: CONTRACT_REVIEW_TIER (narrowing a published accept set). Clause-②: no (narrowing) · Metadata-protocol: none.

Evidence: PR #28302 pins 0.138912345678 stored in full under a declared scale: 4 (home 71987.84); the engine never rounds the rate.

Dedupe: #8345 body ledger; open domain:spec cards searched, none carries this.