[pratikbuilds]

Conversation owns the scroll container so every chat surface scrolls the same way. Scroll policy is a prop: settle lands a sent message at the top and never moves while text streams; stick follows the end while the reader is there. Layout shifts never scroll, and only the reader's own input breaks a follow.

The anchor algorithm and settle geometry are pure functions, and an anchor correction also applies mid-glide so a glide lands where it meant to. useConversation() shares an item registry, scrollToItem, scrollToEnd, captureAnchor/restoreAnchor, the scroll container and a per-frame scroll snapshot that tells the reader's scrolls from the engine's. The context value keeps its identity across scrolls.

Stacked on #1: review only this commit. Part of corbitsdev#94.

Notes for review

• settle is built from the spec text (the reference design only sticks to the bottom); not yet compared side by side with other assistant chat UIs.
• ConversationItem keys by itemId (an id prop would shadow the HTML id).
• Scrolling lives in an imperative controller outside React; the context value is that object, so its identity never changes; atBottom updates at most once per frame.
• Loading history: captureAnchor() before prepend, restoreAnchor() in a layout effect after commit; trimming above needs no calls.
• Both policies start by following the end; settle stops following at the first send or the first scroll away.

Verification

• This commit passes bun run typecheck and bun run lint on its own (the stack is checked commit by commit).
• At the top of the stack: bun run check, bun run build, bun run dep-guard, bun run contrast-test and bun run test:e2e (Playwright specs and the Storybook axe sweep in light and dark) pass.

Closes corbitsdev#99

[pratikbuilds]

Moved upstream: corbitsdev#114