feat(tui): source-back assistant streaming in TUI

[fcoury-oai]

Summary

• render live assistant deltas through a source-backed active markdown cell instead of queued transient history cells
• commit one final source-backed assistant cell on completion, replacing streamed draft text with the final payload
• keep queue-backed commit ticks and stream-time resize follow-up scoped to proposed-plan streaming

Validation

• cargo check -p codex-tui --all-targets
• cargo test -p codex-tui source_backed
• cargo test -p codex-tui streaming
• cargo test -p codex-tui resize_reflow
• cargo fmt --check
• git diff --check