fix: five review findings on the tabbed-sidebar commit

Finding 2 — four builder ids had no null check. `side_stack`,
`side_switcher`, `measure_page` and `signals_page` were resolved at
lines 4709-4712, well past the skeleton null-guard. A typo in the `.ui`
would have been a silent segfault.  The four resolutions are moved to
the guard block and added to its condition, in the same form the guard
already uses.

Finding 3 — the 'editable focused: m refused' leg of keys_probe was
deleted instead of ported.  It is restored, adapted for the new
behaviour: 'm' now selects the Measure tab instead of toggling a pane,
so the refusal is observed as the side_tab name being unchanged across
the synthesised key, not as a toggle that did not flip.  The dump line
keeps its existing name and `refused=` field so probe-readings.md:601
still describes it.

Finding 4 — `exit_probe_step3` assigned `sidebar_shown` to both
`dbg_side_visible` and `dbg_side_column_after_hide`, and the panels:
dump line read the wrong one (dbg_side_visible) for its `(visible=)`
field.  Two readings that cannot disagree diagnose nothing.  Resolved
by making `panels:` read `dbg_side_column_after_hide` and removing the
redundant `dbg_side_visible` field.

Finding 5 — the three-line rationale comment above `reload_file_view`
in `apply_math_channels` was removed by the commit under review.
Nothing it said became untrue.  Restored verbatim from e10a92f.

Finding 6 — three comments the change made false:
- `on_activate`: 'the sidebar's scroller' replaced with 'the sidebar's
  tab switcher and two-page stack', matching the actual ui file.
- `update_side_visibility`: 'Hiding the scroller instead left an empty
  column where the panels should be' removed; the design it described
  no longer exists.
- `header-bar.ui`: 'two of them are toggles' updated to 'one of them
  is a toggle'; after signal_toggle's deletion the overflow menu holds
  one toggle (layout).

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

src/ui/header-bar.ui

@@ -6,7 +6,7 @@
   marks where the *view's* controls end, and the three at the far end.
 
   The bar carries only the controls you reach for *while watching a signal*; everything you set up once lives
-  in the overflow menu, which is code — its rows act on the `RowSet` directly and two of them are toggles
+  in the overflow menu, which is code — its rows act on the `RowSet` directly and one of them is a toggle
   whose state the rest of the interface reads, so a `GMenu` model would only add a second copy of that state.
 
   The order is the file's, and it is the one thing here that is not self-evident: `pack_end()` lays its