site: developer-honest landing page + real repo links

- Replace aspirational marketing with shipped reality: hero terminal and
  CLI showcase now use real Phase 0/1 commands; fictional graphex/trust/
  cost commands moved to a clearly-labelled roadmap block
- Add Project Status (phase tracker) and Internals (under-the-hood)
  sections; status pills on architecture layers (shipped/in-progress)
- Dev-real stats (Rust / 21 crates / BLAKE3 / MIT); honest build-from-
  source quickstart instead of fake `cargo install gpp`
- Wire every link (nav, hero, CTA, footer) to github.com/mahabubul470/gpp
  and docs/ROADMAP.md/ARCHITECTURE.md

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mahabubul470