Fix background block glyph rendering

[askac]

Summary

• add a reusable ANSI terminal rendering test card for full blocks, half-block seams, progress bars, explicit cell backgrounds, box drawing, colors, and wide text
• render Unicode Block Elements as exact canvas rectangles in background terminal PNGs instead of font glyphs
• add pixel-level regression coverage for horizontal, vertical, and half-block boundary seams

Live verification

• foreground visible_xterm_dom: full and half blocks rendered without seams
• background terminal_mirror_canvas: reproduced horizontal and vertical seams before the fix
• explicit blue cell backgrounds confirmed the seam exposed the cell background rather than missing terminal state

Tests

• bash -n scripts/draw_terminal_patterns.sh
• tools/.venv_wsl/bin/python tests/agent_browser_smoke.py
• tools/.venv_wsl/bin/python scripts/run_smoke_tests.py