docs(core): define the internationalization contract - #6048

Problem

Astryx's released internationalization behavior is spread across RFC #3641, consumer docs, tests, and implementation. There is no current repository record that tells reviewers which runtime, fallback, ownership, locale, direction, and calendar boundaries are intentional.

That makes it easy to review a replacement API or locale-sensitive component change without the context that produced the shipped system.

Changes

• Add a draft architecture:internationalization record for the cross-library contract.
• Add a draft component:InternationalizationProvider contract for provider-local behavior.
• Name the provider's nonvisual anatomy in consumer docs so the component record does not imply rendered DOM.
• Add a docs changeset for that consumer-doc addition.

The records preserve the released provider/hook model and the deliberate differences from RFC #3641: client context, deterministic English defaults, silent translation fallback, explicit provider-locale threading, DOM-owned layout direction, and Gregorian PlainDate semantics.

Boundaries and open questions

• Astryx owns Astryx system strings; applications own product copy.
• This does not turn Astryx into a full application i18n framework or admit arbitrary rich-message values.
• Malformed locale tags currently have formatter-specific throw behavior; the draft records that rather than claiming graceful fallback.
• Instant-display utilities that delegate calendar selection to Intl are not treated as first-class alternative-calendar support.
• Nested-provider replacement needs one focused regression test before promotion.
• External runtime adapters and an RSC translation API remain separate API decisions.

This PR is intentionally draft. The records remain authority: draft until their owners approve them.

Test plan

• pnpm check:knowledge
• node scripts/check-changesets.mjs
• pnpm exec vitest run --project node scripts/check-knowledge.test.mjs — 74 tests
• pnpm exec vitest run --project ui packages/core/src/i18n/ — 62 tests
• Verified the provider record against malformed-locale behavior, the current public export barrel, nested-provider source behavior, and PlainDate calendar pinning.