refactor: split tui.ts into focused modules (1,553 → 137 LOC)

Extract monolithic tui.ts into tui-state (381 LOC), tui-tabs (637 LOC),
and tui-renderers (492 LOC) with clean one-directional dependency chain.
Barrel re-exports preserve backward compatibility for all existing imports.
186 new tests across 3 module test files; all 1,337 tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: silouone