docs(spec): define keyboard-reachable scroll containers

[cixzhang]

Why

Keyboard users cannot reach an Astryx-owned scroll container when it overflows but contains no visible sequential-focus control. PR #5553 adds a shared public solution, but no current accessibility contract owns when that fallback stop should exist. The BottomSheet draft also still says focus behavior is unchanged.

What

• Adds draft system spec AST-019 for keyboard-reachable scroll containers: actual per-axis scrollability, descendant ownership, focus stability, author-owned tabindex, visible focus, naming, resource bounds, and browser evidence.
• Keeps assistive-technology verification with AST-009: ordinary Tab order and keyboard scrolling require real-browser evidence, not a real-AT matrix unless the change makes an AT-specific claim.
• Links the BottomSheet draft contract to AST-019 and projects the proposed behavior and verification from fix(core): make text-only Bottom Sheets keyboard-scrollable #5553.

Ordering

This is the proposed shared contract below implementation PR #5553. It remains draft until an authorized owner approves the two decisions and promotes it to current; #5553 should then be reconciled with the accepted exact head.

Validation

• Astryx CLI bootstrap and astryx component BottomSheet --dense
• pnpm check:knowledge -- --base origin/main
• Prettier
• git diff --check

No Changeset: specification records only.

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Project	Deployment	Actions	Updated
astryx	Ready	Preview	Sep 3, 2026 6:42pm UTC

[cixzhang]

Closing at the author's request.