Fix -Ymagic-offset-header for DoubleDefinition error and implicitNotFound errors, and remove global unmanaged cache

[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.

This PR generalizes the header handling logic previous in Positioned.scala and re-uses it in SourcePosition.scala and Symbol.scala, which also need to render source locations

Also turns the global WrappedSourceFile.cache into a per-context cache, so multiple compilers sharing the same classloader don't stomp over each other's wrapped source file header caches (which happens in Mill since we re-use classloaders for performance)

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

Mostly vibe coded with claude

How was the solution tested?

Added new unit tests

Additional Notes

These are issues encountered trying to use -Ymagic-offset-header in Mill to replace the ad-hoc AST/bytecode/error-message mangling com-lihaoyi/mill#5932

[lihaoyi]

This is ready to review, the CI failure is due to a broken master from b2b23b6 CC @tanishiking

[tanishiking]

Oops, there was a conflict between two PRs, sent a patch #25710
Thank you for letting me know :)

[Gedochao]

@lihaoyi Seems there's a conflict now, another rebase necessary