T20: Debug

- DebugToolbar, BreakpointList, CallStack with folded library frames, ThreadList
- VariablesPanel and WatchPanel, colored by kind, changes washed
- HexViewer at a base address with the byte under the pointer lit on both sides
- Disassembly with source lines, registers and numbers colored, gutter breakpoints
- Flamegraph that zooms by frame, TimelineProfiler that zooms and pans by wheel
- NetworkInspector with a waterfall, a filter and a request's headers and body
- icons: redo-dot, arrow-down-to-dot, arrow-up-from-dot
- gallery: the Debug page

TASKS.md

@@ -67,7 +67,7 @@ Per-component lines live in `tasks/`. Tags there:
 - [x] T17e Editor · Status items
 - [x] T18 Terminal
 - [x] T19 Git
-- [ ] T20 Debug
+- [x] T20 Debug
 - [ ] T21a Documents · Editing — `tasks/ch21-30.md`
 - [ ] T21b Documents · Reading
 - [ ] T21c Documents · Knowledge
@@ -245,3 +245,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T19 | 1 | FAIL | a merge rewrote line endings and added a last newline; the split view numbered its right side from the old file; an empty side started its hunk a line late; the subject counter covered long subjects; clipped code rows had no line height |
 | T19 | 2 | FAIL | a lone return split lines for similar but not for regions, so a change was lost; taking both sides added a newline to an empty side |
 | T19 | 3 | PASS | |
+| T20 | 1 | — | pending review |