Add the official ArchAstro Rust SDK

[calvin-archastro]

Summary

• add the generated ArchAstro REST, auth, SSE, and Phoenix channel surface
• add an idiomatic async reqwest runtime with optional blocking methods
• add structured errors, generation-fenced token refresh, and durable app sessions
• add SSE reconnect and Last-Event-ID behavior
• add Phoenix v2 join correlation, heartbeat, reconnect/rejoin, timeout cleanup, stale-join filtering, and buffered pushes
• add rustls/native TLS features, Rust 1.85 MSRV, CI, packaging, and regeneration tooling

Verification

• 13 runtime and focused unit tests
• 1,324 live contracts: 1,300 REST, 5 SSE, 19 channel
• rustfmt and strict Clippy
• warning-free rustdoc
• Rust 1.85 check
• cargo package verification
• npm audit: 0 vulnerabilities
• deterministic regeneration

Depends on ArchAstro/archastro-openapi#72.