feat(markdown): pure-Python port of imgui_md with overlap fix

ADD src/md_renderer_py.py: Full port of mekhontsev/imgui_md to pure Python.
  - Uses markdown-it-py (already a transitive dep) for AST parsing.
  - Walks the token tree, calling imgui primitives directly.
  - Mirrors the C++ API surface: MarkdownOptions, MarkdownCallbacks,
    MarkdownRenderer.render(), render_unindented().
  - Code blocks delegated via set_external_code_block_handler callback.
  - All other content (paragraphs, headings, lists, code, tables, hr,
    emphasis, strong, links, blockquotes) rendered natively.

ROOT CAUSE OF BULLET OVERLAP (now fixed at the source):
  imgui-md C++ BLOCK_P guards NewLine() behind 'if (!m_list_stack.empty())'
  (imgui_md.cpp line ~145). Inside lists, paragraph transitions don't
  advance the cursor Y. The Python port calls imgui.new_line() explicitly
  between paragraphs in a list item, eliminating the overlap.

ROOT CAUSE OF '*' BULLET Y-OVERLAP (now fixed at the source):
  imgui-md C++ BLOCK_LI for '*' delim calls ImGui::Bullet() without
  ImGui::SameLine() (imgui_md.cpp line ~95). The Python port calls
  imgui.bullet() + imgui.same_line() for all markers uniformly.

REMOVED in src/markdown_helper.py:
  - _normalize_bullet_delimiters (no longer needed)
  - _normalize_nested_list_endings (no longer needed)
  - _normalize_list_continuations (no longer needed)
  - parse_tables / render_table (renderer handles tables natively)
  - All 'imgui_md' body rendering (replaced by Python port)

TESTS:
  - tests/test_md_renderer_py.py (new): 16 unit tests for the Python port
    covering paragraphs, headings, lists, nested lists, emphasis, strong,
    code, links, tables, hr, unindented.
  - tests/test_markdown_helper_bullets.py (rewritten): 13 tests for the
    integration with the public MarkdownRenderer class.
  - tests/test_markdown_render_robust.py (updated): 2 tests verifying
    table content is routed through the new Python renderer (not imgui_md).
  - tests/test_markdown_table.py / _render.py / _columns.py / _wrapped.py:
    unchanged (test the standalone render_table which is still used by
    the new renderer as a fallback for any unhandled cases).

42/42 markdown tests pass. 1-space indentation. 1 C++ dependency
removed (imgui_md is no longer used at runtime).

NOT FIXED (known limitations of the new renderer):
  - Inline code rendering uses a tinted small_button (not monospace)
  - Heading fonts use the default font (no separate bold/large fonts)
  - Image rendering shows a placeholder text
  - These can be improved by subclassing MarkdownRenderer

tests/test_markdown_render_robust.py

@@ -1,17 +1,48 @@
-from unittest.mock import patch
+from unittest.mock import patch, MagicMock
 from src.markdown_helper import MarkdownRenderer
 from src.markdown_table import parse_tables
 
-def _mock_table_calls(mock_imgui):
- mock_imgui.TableFlags_ = type("T", (), {"borders": 1, "row_bg": 2, "resizable": 4})()
- mock_imgui.begin_table.return_value = True
- mock_imgui.table_next_column = lambda: None
- mock_imgui.table_next_row = lambda: None
- mock_imgui.table_headers_row = lambda: None
- mock_imgui.text = lambda *a, **k: None
- mock_imgui.end_table = lambda: None
 
-def test_tables_in_crlf_text_all_get_masked():
+def _mock_imgui(mock_imgui):
+ mock_imgui.TableFlags_ = type("T", (), {"borders": 1, "row_bg": 2, "resizable": 4})()
+ mock_imgui.TableColumnFlags_ = type("C", (), {"width_stretch": 8})()
+ mock_imgui.Col_ = type("C", (), {"text": 0, "text_disabled": 1, "frame_bg": 2,
+  "button": 3, "button_hovered": 4, "button_active": 5})()
+ mock_imgui.begin_table = MagicMock(return_value=True)
+ mock_imgui.end_table = MagicMock()
+ mock_imgui.table_setup_column = MagicMock()
+ mock_imgui.table_next_row = MagicMock()
+ mock_imgui.table_next_column = MagicMock()
+ mock_imgui.text = MagicMock()
+ mock_imgui.text_wrapped = MagicMock()
+ mock_imgui.bullet = MagicMock()
+ mock_imgui.same_line = MagicMock()
+ mock_imgui.new_line = MagicMock()
+ mock_imgui.indent = MagicMock()
+ mock_imgui.unindent = MagicMock()
+ mock_imgui.spacing = MagicMock()
+ mock_imgui.separator = MagicMock()
+ mock_imgui.push_style_color = MagicMock()
+ mock_imgui.pop_style_color = MagicMock()
+ mock_imgui.push_font = MagicMock()
+ mock_imgui.pop_font = MagicMock()
+ mock_imgui.small_button = MagicMock()
+ mock_imgui.get_style = MagicMock(return_value=MagicMock(
+  colors={i: MagicMock(x=1, y=1, z=1, w=1) for i in range(6)}
+ ))
+ mock_imgui.get_io = MagicMock(return_value=MagicMock(fonts=MagicMock(fonts=[None])))
+ mock_imgui.calc_text_size = MagicMock(return_value=MagicMock(x=50, y=20))
+ mock_imgui.get_item_rect_min = MagicMock(return_value=MagicMock(x=0, y=0))
+ mock_imgui.get_window_draw_list = MagicMock(return_value=MagicMock(
+  add_line=MagicMock()
+ ))
+ mock_imgui.is_item_hovered = MagicMock(return_value=False)
+ mock_imgui.is_mouse_released = MagicMock(return_value=False)
+ mock_imgui.get_text_line_height = MagicMock(return_value=20.0)
+ mock_imgui.ImVec2 = lambda *a: MagicMock(x=a[0] if a else 0, y=a[1] if len(a) > 1 else 0)
+
+
+def test_tables_in_crlf_text_are_handled_by_python_renderer():
  text = (
   "# Title\r\n"
   "\r\n"
@@ -27,14 +58,15 @@ def test_tables_in_crlf_text_all_get_masked():
  )
  blocks = parse_tables(text)
  assert len(blocks) == 2
- with patch("src.markdown_helper.imgui_md") as mock_md, patch("src.markdown_helper.imgui") as mock_imgui, patch("src.markdown_table.imgui") as mock_table_imgui:
-  _mock_table_calls(mock_table_imgui)
+ with patch("src.md_renderer_py.imgui") as mock_imgui, \
+      patch("src.markdown_helper.imgui_md") as mock_md:
+  _mock_imgui(mock_imgui)
   MarkdownRenderer().render(text, context_id="t")
-  full = "".join(str(c) for c in mock_md.render.call_args_list)
-  for needle in ["| A | B |", "|---|", "| 1 | 2 |", "| X | Y |", "| 3 | 4 |"]:
-   assert needle not in full, f"Raw table text leaked to imgui_md: {needle!r} in calls: {full!r}"
+  assert mock_md.render.call_count == 0, "imgui_md.render must NOT be called — the new design uses the pure-Python renderer"
+  assert mock_imgui.begin_table.call_count >= 2, f"expected at least 2 begin_table calls (one per table), got {mock_imgui.begin_table.call_count}"
+  assert mock_imgui.text_wrapped.called, "body text should still render via text_wrapped"
 
-def test_duplicate_table_content_both_get_replaced():
+def test_duplicate_table_content_both_get_handled():
  text = (
   "| A | B |\r\n"
   "|---|---|\r\n"
@@ -48,9 +80,9 @@ def test_duplicate_table_content_both_get_replaced():
  )
  blocks = parse_tables(text)
  assert len(blocks) == 2
- with patch("src.markdown_helper.imgui_md") as mock_md, patch("src.markdown_helper.imgui") as mock_imgui, patch("src.markdown_table.imgui") as mock_table_imgui:
-  _mock_table_calls(mock_table_imgui)
+ with patch("src.md_renderer_py.imgui") as mock_imgui, \
+      patch("src.markdown_helper.imgui_md") as mock_md:
+  _mock_imgui(mock_imgui)
   MarkdownRenderer().render(text, context_id="dup")
-  full = "".join(str(c) for c in mock_md.render.call_args_list)
-  assert "| A | B |" not in full, f"Raw table text leaked on duplicate: {full!r}"
-
+  assert mock_md.render.call_count == 0
+  assert mock_imgui.begin_table.call_count >= 2, f"expected 2 begin_table calls, got {mock_imgui.begin_table.call_count}"