Tighten SummaryReportWriter test assertions

[goneflyin]

The SummaryReportWriter tests have gaps identified during review:

1. Footer stats values not verified — Footer_ContainsAverageMedianAndLoad checks labels exist but doesn't assert the actual numbers match the input data (e.g. pass crap: 5.0, assert output contains Average CRAP: 5.0).
2. No test for exact column widths — plan says Method 30, Class 35, CC 4, Cov% 7, CRAP 8 but no test verifies row width or cross-row alignment.
3. Coverage formatting edge cases — only tests 100.0%, not 0.0% or mid-range values like 45.5%.

File: tests/Crap4DotNet.Core.Tests/Reporting/SummaryReportWriterTests.cs

Beads ID: crap4dotnet-id3