fix(cy): set abbreviated am/pm to yb/yh instead of inheriting from root

[wildtangent]

Summary

Welsh users formatting 12-hour times (e.g. h:mm a) receive the English strings AM/PM instead of the Welsh abbreviated day-period forms yb (y bore) and yh (yr hwyr). This regressed between CLDR 47 and CLDR 48.

Root cause

In common/main/cy.xml the format/abbreviated am and pm entries were ↑↑↑ in both releases — the data itself did not change. Under ICU 77 (CLDR 47) this was masked by a lateral fallback that resolved ↑↑↑ at the abbreviated width to the locale's own wide entries (yb/yh). ICU 78 (CLDR 48) now resolves ↑↑↑ correctly against the parent locale (root), which has AM/PM, surfacing the gap.

The identifying commit is f81989f8b (CLDR-18850 v48 vxml merge to main), which is the Survey Tool merge that left format/abbreviated am/pm as ↑↑↑.

Changes

• common/main/cy.xml — replace ↑↑↑ with yb/yh in both format/abbreviated and stand-alone/abbreviated contexts, consistent with the existing wide entries.
• common/testData/datetime/datetime.json — add four test cases (hm/hms × AM/PM for locale cy) to prevent silent regression.

Reproduction (Node.js / Intl, which delegates to ICU)

// Pass: Node 24.13.0 / ICU 77.1 / CLDR 47
// Fail: Node 24.16.0 / ICU 78.3 / CLDR 48

const fmt = new Intl.DateTimeFormat('cy', {
  hour: 'numeric', minute: '2-digit', hour12: true, timeZone: 'UTC',
});

const parts = fmt.formatToParts(new Date('2024-07-01T09:30:00Z'));
const period = parts.find(p => p.type === 'dayPeriod')?.value;
// CLDR 47: "yb"  ✓
// CLDR 48: "AM"  ✗
console.log(period);

Diff

 <dayPeriodContext type="format">
   <dayPeriodWidth type="abbreviated">
     <dayPeriod type="midnight">canol nos</dayPeriod>
-    <dayPeriod type="am">↑↑↑</dayPeriod>
+    <dayPeriod type="am">yb</dayPeriod>
     <dayPeriod type="noon">canol dydd</dayPeriod>
-    <dayPeriod type="pm">↑↑↑</dayPeriod>
+    <dayPeriod type="pm">yh</dayPeriod>
     ...
   </dayPeriodWidth>

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[AEApple]

Can you file a ticket? There was some mixed feedback about whether yb/yh are appropriate when we fixed the other locales in 48.2 as part of CLDR-19304 and we should ask the language specialists to review in the Survey Tool while it is currently open for CLDR 49.

We have guidance about requests for data changes in the contributing.md guidelines: https://github.com/unicode-org/cldr/blob/main/CONTRIBUTING.md#tips-for-contributing-to-cldr

If you have evidence of common usage for Welsh it would be really great!