fix(markdown): remove table double-header + add imgui_md bullet workaround

Table fix (src/markdown_table.py):
- Add TableColumnFlags_.width_stretch to each table_setup_column call
  (was missing — columns had no width to wrap against, so text_wrapped
  couldn't grow row height → all rows squished together)
- Remove the explicit for-h-in-headers: table_next_column + text_wrapped(h)
  loop. table_headers_row() already renders the header from the
  table_setup_column() names; the explicit loop was drawing it AGAIN on
  top → double-rendered header rows.

Bullet fix (src/markdown_helper.py):
- Revert _render_md_no_bullet_overlap → simple imgui_md.render(chunk);
  imgui.spacing() (the original af0bbe97 approach). The complex
  workaround was stripping '- ' and rendering stripped text to imgui_md,
  which double-rendered '- 1. ...' content (imgui.bullet from my code +
  numbered list marker from imgui_md).
- Add MarkdownRenderer._normalize_bullet_delimiters: regex-converts
  '* ' markers to '- ' before passing to imgui_md. This works around
  the upstream bug in mekhontsev/imgui_md BLOCK_LI where the '*' case
  calls ImGui::Bullet() without ImGui::SameLine(), causing the bullet
  to render on its own Y with the text on the next Y. The '-' case
  uses Text+SameLine which is correct. Cannot fix from Python (we
  can't subclass the C++ class) — pre-conversion is the cheapest fix.

Tests:
- test_markdown_table_wrapped.py: updated to assert new behavior
  (text_wrapped count == cell count, not header+cell).
- test_markdown_table_columns.py: updated to assert exactly 6
  table_next_column calls (cells only, not 9).
- test_markdown_helper_bullets.py: rewrote for new public-API behavior
  (imgui_md.render called with the unstripped chunk).

16/16 markdown unit tests pass.

src/markdown_table.py

@@ -13,11 +13,8 @@ def render_table(block: "TableBlock") -> None:
  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.table_setup_column(h, imgui.TableColumnFlags_.width_stretch)
  imgui.table_headers_row()
- for h in block.headers:
-  imgui.table_next_column()
-  imgui.text_wrapped(h)
  for row in block.rows:
   imgui.table_next_row()
   for c in row: