Move bench PR comment rendering into the bench tooling

Move the benchmark comment rendering out of uefi-bench-pr-comment.yml and into the step that produces the results, so the comment workflow only downloads an artifact and posts it.

Why

The bencher output format is currently a contract between two places: whatever writes results-*.txt, and a sed/awk parser buried in workflow YAML. A format change breaks the second parser silently.

That script has also grown real logic — field sanitization, removed rows, unit-change detection — which belongs with the tool rather than the CI glue.

Raised by @rogurr in #209.

Acceptance criteria

• Bench step emits the rendered comment body (or a structured artifact) alongside the raw results
• uefi-bench-pr-comment.yml contains no bencher-format parsing
• Rendered comment is unchanged: base/head columns, within-spread labelling, and new / removed / unit changed rows

Dependencies

• Depends on: #209