gh-138122: Integrate live profiler TUI with _colorize theming system

The Tachyon profiler's curses-based TUI now uses the centralized theming
infrastructure in _colorize.py, enabling users to customize colors via
the standard Python theming API. This adds a LiveProfiler theme section
with two pre-configured themes: the default dark theme optimized for
dark terminal backgrounds, and LiveProfilerLight for white/light
backgrounds. Users can switch themes by calling _colorize.set_theme() in
their PYTHONSTARTUP or sitecustomize.py.

The table header rendering was also improved to draw a continuous
background, eliminating visual gaps between columns when using reverse
video styling.

Lib/profiling/sampling/live_collector/widgets.py

@@ -639,8 +639,6 @@ def render(self, line, width, **kwargs):
 
     def draw_column_headers(self, line, width):
         """Draw column headers with sort indicators."""
-        col = 0
-
         # Determine which columns to show based on width
         show_sample_pct = width >= WIDTH_THRESHOLD_SAMPLE_PCT
         show_tottime = width >= WIDTH_THRESHOLD_TOTTIME
@@ -659,38 +657,38 @@ def draw_column_headers(self, line, width):
             "cumtime": 4,
         }.get(self.collector.sort_by, -1)
 
+        # Build the full header line first, then draw it
+        # This avoids gaps between columns when using reverse video
+        header_parts = []
+        col = 0
+
         # Column 0: nsamples
-        attr = sorted_header if sort_col == 0 else normal_header
-        text = f"{'▼nsamples' if sort_col == 0 else 'nsamples':>13}"
-        self.add_str(line, col, text, attr)
+        text = f"{'▼nsamples' if sort_col == 0 else 'nsamples':>13}  "
+        header_parts.append((col, text, sorted_header if sort_col == 0 else normal_header))
         col += 15
 
         # Column 1: sample %
         if show_sample_pct:
-            attr = sorted_header if sort_col == 1 else normal_header
-            text = f"{'▼%' if sort_col == 1 else '%':>5}"
-            self.add_str(line, col, text, attr)
+            text = f"{'▼%' if sort_col == 1 else '%':>5}  "
+            header_parts.append((col, text, sorted_header if sort_col == 1 else normal_header))
             col += 7
 
         # Column 2: tottime
         if show_tottime:
-            attr = sorted_header if sort_col == 2 else normal_header
-            text = f"{'▼tottime' if sort_col == 2 else 'tottime':>10}"
-            self.add_str(line, col, text, attr)
+            text = f"{'▼tottime' if sort_col == 2 else 'tottime':>10}  "
+            header_parts.append((col, text, sorted_header if sort_col == 2 else normal_header))
             col += 12
 
         # Column 3: cumul %
         if show_cumul_pct:
-            attr = sorted_header if sort_col == 3 else normal_header
-            text = f"{'▼%' if sort_col == 3 else '%':>5}"
-            self.add_str(line, col, text, attr)
+            text = f"{'▼%' if sort_col == 3 else '%':>5}  "
+            header_parts.append((col, text, sorted_header if sort_col == 3 else normal_header))
             col += 7
 
         # Column 4: cumtime
         if show_cumtime:
-            attr = sorted_header if sort_col == 4 else normal_header
-            text = f"{'▼cumtime' if sort_col == 4 else 'cumtime':>10}"
-            self.add_str(line, col, text, attr)
+            text = f"{'▼cumtime' if sort_col == 4 else 'cumtime':>10}  "
+            header_parts.append((col, text, sorted_header if sort_col == 4 else normal_header))
             col += 12
 
         # Remaining headers
@@ -700,13 +698,22 @@ def draw_column_headers(self, line, width):
                 MAX_FUNC_NAME_WIDTH,
                 max(MIN_FUNC_NAME_WIDTH, remaining_space // 2),
             )
-            self.add_str(
-                line, col, f"{'function':<{func_width}}", normal_header
-            )
+            text = f"{'function':<{func_width}}  "
+            header_parts.append((col, text, normal_header))
             col += func_width + 2
 
             if col < width - 10:
-                self.add_str(line, col, "file:line", normal_header)
+                file_text = "file:line"
+                padding = width - col - len(file_text)
+                text = file_text + " " * max(0, padding)
+                header_parts.append((col, text, normal_header))
+
+        # Draw full-width background first
+        self.add_str(line, 0, " " * (width - 1), normal_header)
+
+        # Draw each header part on top
+        for col_pos, text, attr in header_parts:
+            self.add_str(line, col_pos, text.rstrip(), attr)
 
         return (
             line + 1,