chore(engine): SVG 栅格化诊断 + first-defect 哈希修正

[leizongmin]

Note

Low Risk
Debug-only append logging on a fixed path; no change to fill injection logic beyond refactoring fill_color_to_css into a local variable.

Overview
Adds temporary SVG rasterization diagnostics in collect_css_svg_presentation: when a computed fill is collected for injection into serialized inline <svg> source, the engine now appends node + resolved CSS fill string to /tmp/t5-svg-fill.log before pushing into the fills list. Rendering behavior is unchanged aside from optional file I/O on nodes with a non-none fill.

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(requestId: be84b8d9-d911-47e5-ae25-0132fa3e26fa)