Fix NativeLinker.LogFailure: add XA3008 coded warning with format arguments

[Copilot]

LogFailure in NativeLinker.ExtractDebugSymbols called log.LogError("Failed to extract debug info", filename, output) — a format string with no placeholders, so the library name and command output were silently dropped. Also lacked an error code and used LogError for a non-fatal condition (debug symbol extraction after successful linking).

Changes:

• Add XA3008 resource string to Resources.resx / Resources.Designer.cs
• Replace LogError with LogCodedWarning("XA3008", ...) using proper format placeholders
• Remove resolved TODO comments
• Add Documentation/docs-mobile/messages/xa3008.md and index entry

// Before: format args silently ignored
log.LogError ("Failed to extract debug info", Path.GetFileName (sourceLib), sb.ToString ());

// After: args properly interpolated into coded warning
log.LogCodedWarning ("XA3008", Properties.Resources.XA3008, Path.GetFileName (sourceLib), sb.ToString ());