Move the Li-native mail stack out of the compiler repo into its own
workspace. Cross-package code references use imports only (declared
deps), never vendored C or cross-repo source inclusion.

- 12 li-mail-* packages under packages/
- Infra deps (li-net, li-tls, li-crypto, li-api, li-rng, li-http)
  resolved via git= URLs to their standalone li-langverse repos
- packages/li.toml workspace manifest (resolver = path)
- Verified: gateway selftest builds with lip install + lic build