fix(mdstream): clamp stable line count to prevent dropping lines shorter than live window

[lx3133584]

Problem

When the rendered output is shorter than live_window, MarkdownStream.update computes a negative num_lines (len(lines) - self.live_window), which causes Python slice lines[num_lines:] to drop leading rendered lines from the Rich live display instead of displaying all lines.

Root Cause

In aider/mdstream.py, num_lines -= self.live_window yields a negative index when len(lines) < self.live_window. Slicing with a negative index takes only the suffix rather than treating the stable line count as 0.

Fix

Clamp num_lines to 0 with max(0, num_lines - self.live_window).

Testing

Added unit tests in tests/basic/test_mdstream.py covering short window preservation and stable line emission, verified with pytest.

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