docs: define React update propagation contract - #6089

Summary

• defines shared React update-propagation boundaries for pre-paint upward writes, membership, mutable metadata, context fan-out, and render isolation
• projects the shared boundary into Stepper through lifecycle-stable membership and focused propagation evidence
• constrains horizontalOptions.minimumStepWidth to finite non-negative pixel numbers; CSS strings, units, calc(), and custom properties are unsupported
• keeps exported StepperContextValue / useStepperContext unchanged from v0.5.2 and moves collapsed-layout coordination to a separate private, non-exported context
• admits private isCollapsed and a parent-owned Collapsed summary render capability while rejecting raw DOM summarySlot exchange
• keeps Frame as named structural anatomy with an intentional none theming disposition and removes astryx-stepper-frame from published target metadata
• records the current hidden threshold probe, added exported context fields, and emitted Frame selector as implementation adoption gaps rather than public contract
• replaces old review-status prose with durable compatibility requirements

Governance

The records define reusable authority. They do not approve, classify, designate, or authorize a specific pull request. Historical pull requests may appear only as evidence.

Verification

• node scripts/check-knowledge.mjs
• pnpm --filter @astryxdesign/core typecheck:docs
• generated astryx component Stepper --dense omits Frame target and shows number-only minimumStepWidth
• Prettier
• git diff --check

No runtime implementation change. No changeset.