The spec claimed the math pane and the signal list were two lists of the same
channels; they are one. `append_math_signals_locked()` already puts every math
channel into `signals()` and nothing filters `sig.math`, so the merge is mostly
a deletion -- the list gains a `MATH` divider and a second menu, and the pane
goes. §2, §3 and §6 say so now.
The plan is seven tasks, each ending in a build, `make test-ui` and one short
headless probe read.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
