[pratikbuilds]

Summary

ConversationMap now renders only while the conversation is taller than its view, so a short chat that fits on screen gets no rail with nowhere to jump to.

• Overflow in the engine: on mount and on every resize of the log or its column, the conversation checks whether the content is taller than the view (contentOverflows in lib/conversation-scroll.ts). The space settle reserves after a sent item does not count, so a fitting chat padded out by its reserve still fits. There is 1px of slack for subpixel rounding.
• Exposed to hosts: useConversation() gains overflowing, and Conversation carries data-overflowing while the content overflows. A host can give the column room for the rail only then, e.g. [&[data-overflowing]_[data-slot=conversation-column]]:pl-12 on Conversation. Both are additive; nothing existing changes shape. Both are false on the server and while no ConversationContent is mounted.
• No flicker: toggling the rail's padding cannot make the map oscillate. Adding the padding makes the content taller when the map appears, and removing it makes the content shorter when the map goes, so each state holds.
• Focus and preview: when the rail goes while a tick has focus, focus moves to the log. A preview that was open does not come back with the rail.
• Story: the FewTurns pane is resizable and pads its column on data-overflowing, so it shows the map coming and going.

Overlap with #145

#145 also moves focus and clears the preview when the map hides (for fewer than two turns). Both touch the same effects in conversation-map.tsx. Whichever lands second needs a rebase; the behaviour is the same.

Verification

• bun run check (891 tests), bun run build and bun run dep-guard pass.
• New tests cover the map hiding and showing as the content grows and shrinks and as the window grows, the reserve not counting as overflow, overflow measured at mount, overflowing resetting when the log unmounts, and focus moving to the log when the rail goes.
• Each new test fails when the behaviour it covers is reverted. With the preview clear removed, the test loops instead of failing.
• Checked in Storybook in Chrome: FewTurns at 480px fits, with no map and 16px of padding. Shrunk to 160px it scrolls, and shows two ticks with 48px of padding. Grown back, both go again. ManyTurns keeps its 150 ticks.

Follow-up in Builder after a release: bump @corbits/react-ui, and move the column padding from data-mapped onto data-overflowing (corbitsdev/corbits-builder#154).

Closes #166

[corbits-triage]

Triage: Pass

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