docs(readme): add Who This Helps and US manufacturing impact sections

[adris-misra]

Summary

Adds two new sections to README.md establishing the framework's national-
interest impact pathway: named beneficiaries and named mechanisms.

What changed

• New section: Who This Helps — SMMs, defense industrial base subcontractors,
  CHIPS-era reshoring brownfield plants, NIST MEP Network, Manufacturing USA
  institutes, academic labs
• New section: How This Advances U.S. Manufacturing — 5 concrete mechanisms
  (knowledge cliff mitigation, AI safety by construction, standards-first
  compliance, OSS multiplier, CHIPS-era supply chain resilience)
• Polish: tightened the Gartner sentence in "Why This Exists" and made the
  benchmarks placeholder more public-facing

Out of scope

Mirroring these sections into docs/index.md for the MkDocs site — separate
follow-up if desired.