feat(tui): integrate table rendering into streaming pipeline with reflow

Wire the Unicode box-drawing table renderer into the live streaming
path with holdback-aware commit logic, post-stream cell consolidation,
and debounced resize reflow.

Key additions:
- StreamCore: shared bookkeeping deduplicating Stream/PlanStreamController
- Table holdback: fence-aware state machine keeps buffer mutable during table detection
- AgentMarkdownCell: stores raw markdown source, re-renders at any width on demand
- ConsolidateAgentMessage: backward-walk replaces streamed cells with single consolidated cell
- Resize reflow: debounced (75ms) re-render of all transcript cells after terminal resize