Fix -Ymagic-offset-header for DoubleDefinition error

[lihaoyi]

This makes the reported error properly use remapped file paths and line numbers, where previously it did not. This was encountered trying to use -Ymagic-offset-header to properly refer to Mill's source .mill scripts during error reporting rather than wrapped generated source files

How much have you relied on LLM-based tools in this contribution?

Mostly vibe coded with claude

How was the solution tested?

Added a new unit test

Additional notes

Root cause: Symbol.sourcePos in Symbols.scala:388-391 returns source.atSpan(span) directly without checking the magic offset header. This means any error reported using
decl.srcPos (which goes through Symbol.sourcePos) — like DoubleDefinition — won't have its position remapped. In contrast, Positioned.sourcePos in Positioned.scala:55-63 does
check WrappedSourceFile.locateMagicHeader and remaps accordingly.

Fix: Added the same magic header check to Symbol.sourcePos:

final def sourcePos(using Context): SourcePosition = {
  val src = if source.exists then source else ctx.source
  WrappedSourceFile.locateMagicHeader(src) match                                                                                                                                   
    case WrappedSourceFile.MagicHeaderInfo.HasHeader(offset, originalFile)                                                                                                         
      if span.exists && span.start >= offset =>                                                                                                                                    
        originalFile.atSpan(span.shift(-offset))                                                                                                                                   
    case _ => src.atSpan(span)                                                                                                                                                     
}                                                                                                                                                                                  

Test: tests/neg/magic-offset-header-e_wrapper.scala — a wrapper with a def foo: String before the marker and def foo: Int after, triggering a DoubleDefinition error. The test
verifies the error position is remapped to the original file.