Optimize renderer incremental redraws

[emil-e]

Summary

• render directly from libghostty terminal pages instead of maintaining a separate RenderState
• batch adjacent dirty-row insertions and preserve renderer-owned buffer positions/scrollback accounting
• resolve styles lazily and keep cursor-only movement updating ghostel--cursor-char-pos
• update render architecture docs, comments, tests, and benchmark/debug tooling

Verification

• make build
• make test-zig
• targeted native ERT tests:
  • ghostel-test-cursor-only-redraw-updates-char-pos
  • ghostel-test-partial-redraw-only-dirty-row-rebuilt