Use PR target branch as synthesis diff baseline

[Copilot]

Synthesis metrics diffs were still framed around main, which is incorrect for PRs targeting non-main branches (e.g. multicycle). This change makes the diff/report context track the PR base branch.

• Workflow baseline context

  • Added BASE_BRANCH: ${{ github.base_ref }} to synthesis diff jobs in ci.yaml so scripts receive the PR target branch explicitly.

• Per-metric diff script updates

  • Generalized compare_metrics_diff.sh from main-specific naming/messages to base-branch naming.
  • Updated fallback text to report missing baseline for the actual target branch rather than hardcoded main.

• Aggregated report wording

  • Updated aggregate_metrics.sh footer text to say comparison is against $BASE_BRANCH instead of main.

# .github/workflows/ci.yaml
env:
  BASE_BRANCH: ${{ github.base_ref }}

# compare_metrics_diff.sh / aggregate_metrics.sh
BASE_BRANCH="${BASE_BRANCH:-base branch}"
echo "*Comparing synthesis results from \`$BASE_BRANCH\` branch vs. this PR*"