Hide editor and show range indicator during history scroll

[shellicar]

Summary

• Editor collapses during history scroll, giving history the full screen
• Position indicator shows visible range (e.g. [↑ 1-24/52]) instead of single line

Related Issues

Closes #154
Closes #155

[bananabot9000]

Two issues (#154 + #155) in one clean PR.

Zone collapse: Early-return branch in renderZone() gives history screenRows - 1 rows, zone collapses to 1 indicator row. History viewport resolved first so indicator reflects current position on the same frame. No one-frame lag.

Range indicator: start-end/total with Math.min cap on end for short buffers. Separate test verifies 1-10/10 not 1-24/10.

wrappedHistory hoist: displayBuffer.flatMap(wrapLine) moved above the history/live branch so both paths share it. Correct.

6 new tests: history mode row allocation, returnToLive, range math (large + short buffer), scrollback violations with collapsed zone.

The PgDown-in-live-mode rendering bug (stale cells not overwritten) is a separate concern -- likely needs the renderer to clear/pad rows when zone expands back from collapsed state.

LGTM.

Co-reviewed-by: BananaBot9000