fix(markdown): add missing table_setup_column calls in render_table

ROOT CAUSE: src/markdown_table.py:render_table was missing
imgui.table_setup_column() calls. In ImGui, columns MUST be
configured via table_setup_column before table_headers_row is called.
Without it, the table has no defined columns, causing cells to
render at overlapping Y positions. This manifested as text overlap
in the Discussion Hub's read_mode entries (e.g., 'swc2 -> gte_sw'
overlapping the line above it).

FIX: Call imgui.table_setup_column(h, TableColumnFlags_.width_stretch)
for each header BEFORE table_headers_row(). Each column now has a
defined width (stretch = fills available space) and cells render
correctly without overlap.

Tests:
- New test_markdown_table_columns.py asserts setup_column is called
  once per column and table_next_column is called for each cell.
- 16/16 broad regression pass (test_markdown_table,
  test_markdown_table_render, test_markdown_render_robust,
  test_gen_send_empty_context, test_gui_fast_render)

src/markdown_table.py

@@ -12,6 +12,8 @@ def render_table(block: "TableBlock") -> None:
  if n_cols == 0: return
  flags = imgui.TableFlags_.borders | imgui.TableFlags_.row_bg | imgui.TableFlags_.resizable
  if not imgui.begin_table("md_table", n_cols, flags): return
+ for h in block.headers:
+  imgui.table_setup_column(h, imgui.TableColumnFlags_.width_stretch)
  imgui.table_headers_row()
  for h in block.headers:
   imgui.table_next_column()