Add the bounded output window

• Status: Todo
• Kind: Feature
• Authors: Jean Mertz git@jeanmertz.com
• Date: 2026-08-27
• Implements: 091

Adds the bounded line buffer with coalesced redraw, source registration and labelling, the SGR filter, ANSI-aware truncation, terminal height as a capability input, multi-row draw/erase with relative row accounting, and the print_stderr config shape (splitting ProgressConfig). Opens with a spike against a real terminal to settle cursor-at-last-row, concurrent-write, and resize/Windows cases, whose output pins Printer::memory regression tests; no external client uses a non-zero window yet.