ui: the signals panel's frame is a .ui file

src/ui/signals-panel.ui holds the panel's shape: the box and its margins, the heading,
the hint that says what a signal is for, and the list. The rows stay in code -- one per
signal, carrying that signal's switch, colour, calibration and drop targets, rebuilt
whenever the stream's signals change -- so the file holds the frame, which is the shape
the plan gives this slice.

The list's width-only size request moved with it, comment and all: a height floor there
made the list occupy 120 px whatever it held, which hid the compact rows' height behind
it.

Read back by a new dump line, `signals panel: labels=2 list=1`, where a lost id reads as
`labels=1` or `list=0` instead of a panel quietly missing its hint; `panels: … row_h=26`
still reads the list's geometry and the height a row takes. check_ui is at 10 ok.

docs/claude/invariants-gtk-and-chrome.md

@@ -309,3 +309,10 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   Related: `gtk_expander_set_child()` asserts that its child has no parent yet, so a block the `.ui` has
   already put in the panel cannot then be handed to an expander -- the file's `child` property is the right
   way, and it works.
+
+- **`--` is not allowed inside an XML comment, and a `.ui` comment is XML.** `<!-- ... -- ... -->` is a parse
+  error: `xml-stripblanks` fails the build, and `make test-ui` prints `FAIL <file>` with **no** message under
+  it, because blueprint-compiler's decompiler aborts on the malformed XML before it can report anything --
+  the bare `FAIL` line is that mistake and nothing else. The house style for a dash in prose is the em dash,
+  which is why no other `.ui` in `src/ui/` contains a double hyphen. It has cost two slices a debugging
+  round for one character.