fix: a repaired source is reconciled above the validation line, not below it

`math_dialog_sync_form()` wrote the label and then moved a node whose source
names an unacquired channel -- a mutation that changes what
`math_dialog_problem()` reports. With the sync re-entry now guarded, the nested
call that used to repair the label is suppressed, so the line kept describing a
source that had just been fixed and left `Apply` insensitive until the next
mutation. The reconcile moves above the write and the dropdown reuses its list.

docs/claude/invariants-math-channels.md

@@ -25,7 +25,8 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   two-click protocol is gone), drag a box to move it; **wheel zooms about the pointer, background
   drag pans, `f` fits, `Ctrl+0` resets**.
   Its widgets are created once and only shown, hidden and set by `math_dialog_sync()`, so no rebuild
-  can invalidate a live callback handle -- the same shape the measurements dialog uses. `UDPSCOPE_PROBE_MATH_MS` defines two graphs and opens the editor (a construction crash test
+  can invalidate a live callback handle -- the same shape the measurements dialog uses.
+  `UDPSCOPE_PROBE_MATH_MS` defines two graphs and opens the editor (a construction crash test
   as much as a state check), and the dump prints each derived channel's extremes beside the
   source's. Read it on the fast mock, where the two assertions that matter are **`gaps=0`** (the
   math is on the producer thread, so what it can break is packet capture, never the frame rate)