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Add curses/fullscreen app support: alt buffer, missing CSI sequences, parsing fix

Curses-based applications (e.g. NetHack, vim, htop) rely on several CSI
escape sequences and terminal features that were not previously handled,
causing rendering artifacts such as old screen content showing through
popup windows and garbled escape sequences displayed as literal text.

New escape sequences:
- ECH (CSI Ps X): Erase Character — blanks Ps characters from cursor
- DL (CSI Ps M): Delete Line — removes lines within scroll region
- SD (CSI Ps T): Scroll Down — scrolls region down, inserts blanks
- REP (CSI Ps b): Repeat — repeats preceding graphic character Ps times
- Window ops (CSI Ps;..t): title save/restore — accepted as no-op

Alternate screen buffer:
- Implement enterAltBuffer/exitAltBuffer for CSI ?1049h/l and ?47h/l
- Deep-copies grid content on enter, restores on exit
- Prevents curses app content from corrupting the main screen

Fix cross-buffer escape sequence parsing:
- Replace position-based ESC detection (t.state.esc == i-1) with a
  boolean flag (escNext) that persists correctly across read buffer
  boundaries. Previously, when ESC was the last byte of one read and
  '[' was the first byte of the next, the CSI sequence was not
  recognised, causing the parameters (e.g. "0;33H") to render as
  literal text on screen.

Other fixes:
- CSI 2K (erase entire line) now clears the row instead of filling
  with zero-valued cells
- Always call Refresh() after processing output, not only when there
  is no leftover data — fixes missed redraws during data bursts
- Handle private modes: DECCKM (?1), blink (?12), empty mode

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>