Feature/font logging

[swmal]

Font logging

Adds diagnostics that explain which fonts and glyph fallbacks are chosen.

Usage

Set Logger inside ConfigureFonts. It is null by default (no logging) and cleared by Reset(). Already cached fonts are not resolved again, so set it inside the callback.

workbook.ConfigureFonts(c =>
{
    c.Logger = FontLoggerFactory.CreateTextFileLogger(new FileInfo(@"c:\fontlog.txt"));
});

CreateTextFileLogger writes one line per event (DBG/INF/WRN/ERR), overwrites an existing file and needs no disposal.

Custom logger

IFontLogger has two members: IsEnabled(FontLogSeverity), called before an event is built, and Log(FontLogEvent).

class MyLogger : IFontLogger
{
    public bool IsEnabled(FontLogSeverity s) => s >= FontLogSeverity.Warning;
    public void Log(FontLogEvent e) => Console.WriteLine(e.Message);
}

workbook.ConfigureFonts(c => c.Logger = new MyLogger());

FontLogEvent has a Message plus structured fields (Type, RequestedFont, ResolvedFont, Script, CodePoint). Implementations must be thread-safe; exceptions are swallowed. Glyphs found in the primary font are not logged.