Fix scrollback corruption when editor exceeds terminal height

[shellicar]

Summary

Complete rewrite of the terminal functionality.

Terminal no longer writes more rows than screen.rows, eliminating scrollback corruption when the editor exceeds terminal height.

Changes

• Layout/Viewport/Renderer pipeline replaces buildSticky()/clearStickyZone()
• Viewport bounds every frame to screen.rows with cursor chasing on scroll and resize
• MockScreen test double detects scrollback violations for automated verification

Related Issues

Closes #146

Co-Authored-By: Claude noreply@anthropic.com

[bananabot9000]

The full terminal rendering redesign from Issue #146, implemented across 5 micro-session stages. 1,307 additions, 167 tests, matches the design doc.

Architecture verified:

• Screen interface + StdoutScreen + MockScreen (with ANSI interpreter and scrollback violation tracking)
• Layout -- pure function, correct component ordering, wrapLine with stringWidth
• Viewport -- 31 lines, cursor chasing, resize capping
• Renderer (as TerminalRenderer.ts -- smart macOS case collision avoidance) -- sync markers, zone-aware rendering
• terminal.ts -- buildSticky()/clearStickyZone() replaced with renderZone() pipeline through Layout > Viewport > Renderer

Bugs caught during Stage 5 (good catches):

1. Viewport padding caused scrollback violations on first render when cursor was not at row 0 -- fixed by trimming trailing empty rows before writing
2. writeHistory re-rendered stale lastFrame with cleared question still visible -- fixed by splitting into writeHistoryLine + fresh renderZone()

Minor notes (non-blocking):

• wrapLine iterates by code point not grapheme cluster (Intl.Segmenter would handle compound emoji at wrap boundaries). Not a regression.
• Deprecated writeHistory on Renderer -- can it be removed if only writeHistoryLine + external render is used?

The one invariant holds: the renderer never writes more rows than screen.rows. Scrollback corruption is now structurally impossible.