badges: hide the Source badge when the source has no panel of its own

A Source pill that opens nothing (UDPS, Flat UDP, files) was noise. It is now shown only while the slide-over has a source page; the [badges] source line gains visible=.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/claude/invariants-plot-and-layout.md

@@ -117,7 +117,7 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   (`height-request`) between `plot_overlay` and the status line, so the plot above it never moves when a
   badge's text changes (`bar_h` = `bar_h_after` in `[badges]`). This is "layout must not depend on
   content" for the new row: the Trigger badge's state is a word from a short fixed set and the Source badge
-  is a kind's display name only (no connection state: that is the status bar's), so what changes with the stream is the row's *width* (the bar has room to
+  (shown only when the kind has source pages; hidden otherwise) is a kind's display name only (no connection state: that is the status bar's), so what changes with the stream is the row's *width* (the bar has room to
   spare on the right) and never its height. The badge labels are not ellipsized: a name long enough to
   need it would be a defect of the kind's `display_name()`. Rows that appear by trigger condition appear
   inside the slide-over's scroller, never in the bar.
@@ -127,8 +127,8 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   (the E-1608: eight inputs and the counter) would push the window's minimum width past a laptop screen.
   Past `kMaxSignalBadges` (6) the row is replaced by one "N signals" badge, which also stands for the `+`.
   The bar's height stays 36 px in every one of these states (`bar_h_fixed` in `[sigbadges]`), and
-  `bar_min_w` reads the same with one signal, nine signals and the overflow (244 on the E-1608, 251 on UDPS:
-  the Source badge of each kind reads differently). A change that makes it grow with the signal count has
+  `bar_min_w` reads the same with one signal, nine signals and the overflow (244 on the E-1608, 124 on UDPS:
+  the Source badge is on screen for the E-1608 and hidden for UDPS, and its text differs by kind). A change that makes it grow with the signal count has
   taken the scroller out of the measurement.
 - **The dock's width is fixed too** (`side_column` `width-request` 300, the split view's 320 px floor
   and cap), and it holds one scroller with the cursors and the measurements. A