fix(coverage): detect minority-locale translations and honor --doc-locale

[dacharyc]

Summary

• detectLocalePosition only recognized multi-locale prefixes when locale codes covered >50% of sitemap URLs, so sites with an unprefixed default language and minority translations (Datadog's /fr/, /ja/, /ko/, /es/ at ~42%) kept translated pages in the llms-txt-coverage denominator
• Generalizes the structural-duplication fallback from the single-code case to any number of locale codes: a position counts as a locale position if stripping one of its codes produces paths that match unprefixed URLs in the same set, so locale-like topic segments (e.g. /it/ for IT docs) still aren't filtered
• Honors an explicit --doc-locale / preferredLocale in the coverage check: it wins over inference from llms.txt URLs, and a preferred locale with no prefixed sitemap URLs is treated as the unprefixed default
• Documents locale handling in docs/checks/observability.md

Closes #98

Test plan

• Unit tests for detectLocalePosition (majority, minority-with-duplication, locale-like-without-duplication, none)
• Integration tests: Datadog-style minority-locale filtering, topic-segment non-filtering, explicit --doc-locale (prefixed and unprefixed-default)
• npm test (1303 tests), npm run lint, prettier clean