fix(markdown): strip backticks in table cells + add nested-list overlap workaround

FIX 1 (src/markdown_table.py): Cells now use imgui_md.render(c) instead of
imgui.text_wrapped(c). imgui_md uses MD4C which strips backtick-delimited
inline code spans BEFORE rendering, so backticks no longer appear as
literal characters in cell content. Side benefit: inline emphasis
(*foo*, **bar**) now renders in cells too.

FIX 2 (src/markdown_helper.py): Added MarkdownRenderer._normalize_nested_list_endings.
Upstream imgui_md (mekhontsev/imgui_md) BLOCK_UL exit only calls
ImGui::NewLine() for top-level list endings. For nested list endings, no
NewLine is emitted, so the next text starts at the same Y as the last
list item, causing visual overlap. The preprocessor inserts a blank
line before any line that follows a list item with MORE indent than
itself, forcing a paragraph break. Cannot fix the C++ from Python.

Tests:
- test_markdown_table_wrapped.py: updated to assert imgui_md.render is
  called for cell content (not imgui.text_wrapped).
- test_markdown_helper_bullets.py: added 4 tests for the new preprocessors
  (nested-list blank insertion + bullet delimiter conversion + edge cases).

20/20 markdown unit tests pass. 1-space indentation throughout.

KNOWN LIMITATIONS (cannot fix without forking imgui_md C++):
- Inline code spans render as plain text (no monospace font in cells)
- The ' * ' bullet delimiter has a Y-overlap bug upstream
  (workaround: pre-convert to '- ' via _normalize_bullet_delimiters)
- Nested list ending overlap (workaround: insert blank line via
  _normalize_nested_list_endings)

src/markdown_table.py

@@ -1,6 +1,6 @@
 import re
 from dataclasses import dataclass
-from imgui_bundle import imgui
+from imgui_bundle import imgui, imgui_md
 
 _TABLE_SEPARATOR = re.compile(r"^\|?\s*:?-{2,}:?\s*(\|\s*:?-{2,}:?\s*)+\|?\s*$")
 
@@ -19,7 +19,7 @@ def render_table(block: "TableBlock") -> None:
   imgui.table_next_row()
   for c in row:
    imgui.table_next_column()
-   imgui.text_wrapped(c)
+   imgui_md.render(c)
  imgui.end_table()
 
 @dataclass(frozen=True)