Add headline results and fix pack dry-run CI

[ethanj]

Summary

• add the headline benchmark results table to the monorepo README before the product positioning section
• update the README performance posture copy so it points readers back to the headline results without overclaiming latency
• run the Turbo build graph before package dry-run validation so clean CI checkouts have dependency build output before npm pack invokes package prepack scripts

Validation

• pnpm run ci:pack-dry-run
• git diff --check