[pratikbuilds]

ConversationMap shows one tick per turn, marked faint, on screen or current, previews a turn on hover or focus and jumps to it with the conversation's glide. The rail is one tab stop with arrow keys and Enter, and keeping the current tick in view scrolls only the rail. useReadingPosition reports the current and on-screen turns from one IntersectionObserver over the item registry, measuring on the conversation's per-frame scroll snapshots. A jumped-to turn stays current until the reader scrolls.

Stacked on #10: review only this commit. Part of corbitsdev#94.

Notes for review

• useReadingPosition reads the container from useConversation().scrollContainer() and measures on subscribeScroll snapshots; works when a host overrides ConversationContent's data-slot.
• jumpTo pins a clicked turn as current until the reader actually scrolls (a byReader snapshot with movement); clicking or typing inside the log keeps it. The keyboard preview follows its tick along an overflowing rail.
• Keep-in-view scrolls only the rail (rail.scrollTo); the page never moves (e2e asserts window.scrollY stays 0).
• A turn whose start item isn't mounted (windowed history) is never on screen; clicking its tick only fires onSelect.
• (04) MessageSendStatus and MessageHeader register through Message's context, so they work at any depth (e.g. inside MessageFooter) and Message never doubles the status line. The live region belongs to Message and reads the current error (or "Not sent."). Server-rendered HTML shows the default line until hydration registers a placed one.

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#101

[pratikbuilds]

Moved upstream: corbitsdev#123