chore(release): prepare dshell v0.6.1

[metah3m]

Summary

• promote dshell from 0.6.1-beta.1 to stable 0.6.1
• move the network stack to stable h3x 0.6.1, resolving to dquic 0.7.1, qconnection 0.8.1, and qresolve 0.8.0
• update the direct base64 dependency to stable 0.23
• keep remoc 0.18.3 because h3x 0.6.1 uses the 0.18 RPC types; forcing 0.19.1 creates incompatible duplicate remoc types and does not compile

Verification

• cargo fmt --check
• cargo clippy --all-targets --features server,client,cli,config -- -D warnings
• cargo test --features server,client,cli,config -- --skip session::process::tests::run_pty_sends_exit_status_before_late_output_but_still_drains_pty (227 passed; skipped test also fails on the unchanged beta baseline on macOS; Ubuntu CI runs the complete all-feature suite)
• cargo publish --dry-run --allow-dirty
• cargo update --dry-run --verbose reports no compatible updates; only incompatible remoc 0.19.1 remains

No merge, tag, or crates.io publish is included in this PR.