Start a busy-pin-polarity log, captured as panels are touched

Not project scope on its own - would need every controller's datasheet to
explain, not just record - but cheap to note per-panel since we're already
reading each vendor constructor for the descriptor work.

components/gxepd2/busy_pin_polarity.md

@@ -0,0 +1,20 @@
+# Busy pin polarity, by panel
+
+Not a project scope item on its own - GxEPD2 doesn't record *why* a given
+panel/controller is active-high or active-low on BUSY, and pinning that
+down for every panel would mean reading every controller datasheet. This
+file just captures what we've read off each vendor driver constructor's
+`busy_level` argument to `GxEPD2_EPD(...)` as we happen to touch a panel
+for other reasons - `EpdPanelDescriptor::busy_active_level` in this
+component encodes the same fact per-panel (`true` = active-high).
+
+| Panel | Controller | busy_level (vendor ctor) | busy_active_level | Hardware-confirmed? |
+|---|---|---|---|---|
+| GDEY0213Z98 | SSD1680 | HIGH | true | yes (bench panel, working) |
+| GDEQ0426T82 | SSD1677 | HIGH | true | yes (2026-09-19) |
+| GDEQ0583T31 | UC8179 | LOW | false | pending |
+| GDEY029F51 | JD79667 | LOW | false | no (4-color, not yet ported to this engine) |
+| GDEY0579F51 | HX8717 | LOW | false | no (4-color, not yet ported to this engine) |
+
+Source: each driver's own `.cpp`, `GxEPD2_EPD(cs, dc, rst, busy, <level>, ...)`
+constructor call (grep `GxEPD2_EPD(cs, dc, rst, busy` in `vendor/src/**/*.cpp`).