refactor: simplify async host execution

[fffonion]

Summary

• convert HTTP, SSE, SQLite, and async I/O hosts to ordinary macro-generated async functions using the generic host-future lifecycle
• use Hyper/hyper-util connection management and pooling, tokio-rusqlite, and Tokio-backed I/O without host-owned runtimes, thread pools, mailboxes, or operation drivers
• enforce incremental SSE callback backpressure, cancellation-safe SQLite close/transaction deadlines, retryable process cleanup, complete bridge quiescence, and nonblocking Drop paths
• keep only the current host catalog/fingerprint and VMBC v14 implementation; remove historical compatibility, dedicated rejection, migration fixtures, and source-scanning architecture guards
• update current HTTP and Host SDK documentation

Validation

• cargo fmt --all -- --check
• workspace all-target/all-feature check
• workspace Clippy with -D warnings
• workspace all-feature tests: 2,298 passed, 12 ignored
• complete feature/lifecycle matrix: 10,050 passed, 0 failed, 60 ignored
• no-default, runtime-only, HTTP-only, SQLite-only, pd-vm-nostd, thumbv6m, wasm32, and Windows compile gates
• independent scoped reviews and whole-change review completed cleanly