Theme Polish & Tone Mapping

Problem

Missing Theme Colors: The ThemePalette dataclass in src/theme_models.py only defined a subset of the ~55 ImGui colors. Because from_dict strictly matched dataclass fields, colors like resize_grip and tab_dimmed from the TOML files were being discarded, breaking window resizing handles and inactive tab styling.
Context Preview Syntax Palette: theme_2.apply() failed to apply the syntax palette for non-NERV themes, and src/markdown_helper.py cached its TextEditor instances without clearing them on theme switch. This caused "Context Preview" to remain stuck on the previous theme's syntax colors.
Light Theme Brightness: The user requested a way to dim light themes. We will introduce a Tone Mapping system (Brightness, Contrast, Gamma) that mathematical adjusts the RGB colors before applying them to ImGui. The user requested this to be saved per-palette so each theme can have its own exposure profile.

Ensure src/theme_models.py's ThemePalette dataclass has all missing ImGui colors (e.g., resize_grip, resize_grip_active, resize_grip_hovered, tab_dimmed, tab_dimmed_selected, docking_preview, plot_lines, nav_windowing_highlight, etc.). (Note: I proactively applied the class definition update during exploration, but will formally commit it).

Update src/theme_2.py to ensure apply_syntax_palette() is called for all themes during apply().
Add an import src.markdown_helper; src.markdown_helper.get_renderer().clear_cache() call to the end of theme_2.apply() to force code blocks to recreate their TextEditor instances with the new palette.

Add mathematical tone mapping variables to src/theme_2.py: _brightness, _contrast, and _gamma (stored as dictionaries keyed by the palette name to allow per-palette saving).
Implement a math function to adjust RGB floats:
- Brightness: c * brightness
- Contrast: (c - 0.5) * contrast + 0.5
- Gamma: pow(c, 1.0 / gamma)
Update the palette application loop in theme_2.apply() to pass every color float through this tone mapper before calling style.set_color_().
Update save_to_config and load_from_config to persist the tone mapping overrides per-palette under [theme.tone_mapping.<palette>].
Add Brightness, Contrast, and Gamma sliders to the Theme panel in src/gui_2.py.

Model & Sync Fixes: Verify src/theme_models.py and update src/theme_2.py's apply() function to trigger syntax updates and markdown cache clearing.
Tone Mapping Logic: Add the dicts and the math _tone_map(rgb, palette) function to theme_2.py, wrapping all color assignments.
State Persistence: Update save_to_config / load_from_config to handle the new per-palette dictionary.
UI Integration: Add the 3 sliders to _render_theme_panel in src/gui_2.py, complete with a "Reset to Defaults" button for the current palette.
Testing: Run the existing test suite and verify no regressions in config saving.