feat(http): add callable streaming clients

[fffonion]

Summary

• add typed callable host schemas and a generic one-item-at-a-time host stream pump
• add bounded callable-driven SSE and WebSocket clients with shared connection, security, and total-deadline policies
• preserve VM provenance, callback serialization, invocation abandonment cleanup, backend parity, feature gating, and capability isolation
• document the Rust embedding API and script-facing streaming contracts

Stack

Layer 14 of 14
Previous: #25

Validation

• cargo test --workspace --all-features — 1366 passed, 13 ignored
• cargo test -p pd-vm-nostd — 35 passed
• focused SSE, WebSocket, host-stream, invocation-stream, binding, feature-gating, and alias suites
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --all-features --no-deps
• cargo fmt --all -- --check
• runtime-only no-default check, workspace no-default test compilation, and wasm32 release build
• 55 repeated timing-sensitive SSE/WebSocket runs with 0 failures