Anti-aliased line/circle rendering and grayscale PNG output

[Grizaster]

Overview

This PR improves rendering quality by adding anti-aliasing for all line and circle primitives, and switches PNG output from 1-bit monochrome to 8-bit grayscale (matching Labelary's default X-Quality: Grayscale output).

Rendering Improvements

Anti-aliased diagonal lines (^GD)

• Thin lines (thickness ≤ 1): Replaced Bresenham (draw_line_segment_mut) with Xiaolin Wu's algorithm (draw_antialiased_line_segment_mut) for smooth edges
• Thick lines (thickness > 1): Custom parallelogram fill with per-pixel coverage at the left and right edges. The previous draw_polygon_mut approach only drew the outline, not the filled shape

Anti-aliased circles (^GC)

• Replaced draw_filled_circle_mut and the binary distance-check ring with a per-pixel coverage-based approach
• Both inner and outer ring edges use a 1px-wide coverage ramp (distance ± 0.5) for smooth anti-aliased edges

Grayscale PNG output

• Removed the binary threshold (pixel > 128 ? 255 : 0) from encode_png
• PNG output is now 8-bit grayscale, preserving anti-aliasing from the renderer
• Matches Labelary's default X-Quality: Grayscale output

Tests

• Updated unit_png_encoder tests: assert grayscale passthrough instead of binary threshold behavior
• Fixed clippy manual_clamp warnings: use clamp() instead of max().min()
• Increased dhlparceluk golden test tolerance from 5.5% to 6.0% (5.57% diff due to anti-aliasing)
• Regenerated all testdata/diffs/ after rendering changes

Files Changed

• src/drawers/renderer.rs — anti-aliased line and circle rendering
• src/images/monochrome.rs — grayscale PNG encoding (removed binary threshold)
• tests/unit_png_encoder.rs — updated tests
• tests/e2e_golden.rs — tolerance adjustment
• docs/DIFF_THRESHOLDS.md — updated tolerance
• testdata/diffs/* — regenerated diff images and reports