fix: preserve instance scope when resolving fallback locales

[OskarEichler]

Fixes

• Keep regional locale fallback local when invoked through an instance or a local-only locale lookup. dayjs().locale('en-US') must not change the global locale from fr to en.
• Keep construction safe when a requested locale is unavailable: use the current global locale rather than leaving $L undefined. This preserves the previous unknown-regional fallback and also handles unsupported non-regional constructor locales consistently.

The recursive parseLocale call previously dropped isLocal. Forwarding it fixes the global mutation; the constructor fallback prevents a new formatting failure when neither the region nor its base language is registered.

Reproduction

const dayjs = require('dayjs')
require('dayjs/locale/fr')
dayjs.locale('fr')
dayjs('2026-08-27').locale('en-US')
console.log(dayjs.locale()) // before: en; after: fr

Compatibility

No public API, type, dependency, or minimum-runtime changes. Explicit global dayjs.locale('en-US') still changes the global locale. Existing instances and date values remain unchanged. Unknown instance-local locales retain that instance's locale. Code relying on the accidental global mutation must call the global setter explicitly; unsupported constructor locales now safely use the global default.

Verification

• All 794 existing tests in 93 suites; 100% line coverage.
• Existing timezone and timezone-plugin matrices (eight runs).
• Lint, all 40 declaration files, ESM/UMD/plugin/locale builds, and size limit.
• 29 focused source checks covering regional/case/unknown locales, construction, local/global registration, immutability, UTC, relative time, and custom-format parsing followed by locale selection.
• Repeated the existing suite and builds on release 1.11.23 with the backport.

No test files were added or modified. The old size-limit Webpack dependency needs NODE_OPTIONS=--openssl-legacy-provider under Node 22; that was a verification-only setting, not a package change. Existing Handlebars coverage-report and Kurdish mixed-export warnings remain. CustomParseFormat's separate direct regional-language lookup is unchanged.