feat: introduce warnings/traces fields and optional stderr prefix of attribute path

[ConnorBaker]

Caution

This is a work in progress and entirely vibe-coded.

The goal of the PR is to make it easier to analyze nix-eval-jobs logs for both people (i.e., by providing attribution of warnings/traces to the attribute paths which created them) and scripts (i.e., adding warnings/traces fields to the JSON objects being streamed out).

Summary

• Worker subprocesses now wrap nix::logger with a PrefixLogger decorator that can prepend attrPath: to all stderr log lines during evaluation, making interleaved multi-worker output attributable
• The stderr prefix is opt-in via --log-attr-prefix to avoid breaking existing scripts that parse stderr
• Evaluation log messages are buffered per-attribute and exposed as optional traces and warnings fields on the JSON output (omitted when empty), giving machine consumers access to builtins.trace and builtins.warn output that was previously only available on stderr
• builtins.trace output goes to traces; builtins.warn and structured warnings from logEI go to warnings; errors remain in the existing error field

[ConnorBaker]

@Mic92 how does this approach compare with the one you've got in #396? I see both introduce a new logger, but yours additionally introduces attachTracesToLastWarning. (The name of the logger in this PR has grown rotten, given that it's also saving warnings/traces in addition to optionally prefixing logs.)