docs(mistakes): record the record-struct zero-init trap

Two controls looked dead because new ScoreboardStyle() zero-initialised instead of applying the
declaration's defaults. Recorded with the rule (explicit parameterless ctor) and the testing lesson
(a defaults test must construct with new(), not with explicit arguments).

MyMistakes.md

@@ -1,3 +1,23 @@
+
+## 2026-10-30 — C# record struct: `new T()` ignores the parameter defaults
+
+**Symptom:** two property controls "had no code behind them" — the Background checkbox and the
+opacity slider changed nothing visible.
+
+**Cause:** `public readonly record struct ScoreboardStyle(string TextColor = "#FFFFFF", bool BackgroundEnabled = true, …)`.
+`new ScoreboardStyle()` binds to the implicit **parameterless constructor**, which zero-initialises —
+the defaults in the parameter list are only used when the constructor is called with those arguments
+omitted *at a call site that supplies some of them*. Result: null colour, `BackgroundEnabled = false`,
+`FontOpacity = 0`, `FontSize = 0`. The renderer drew the zeros faithfully, so the UI looked dead
+while every unit test of the renderer passed (they constructed the style explicitly).
+
+**Rule:** a record struct with meaningful defaults needs an explicit parameterless constructor
+(`public T() : this(<defaults>) { }`). And a "defaults" test must construct the type the same way
+production does — with `new()` — never with explicit arguments, or it proves nothing.
+
+**Guard:** `ScoreboardRendererTests` asserts `new ScoreboardStyle()` yields white / on / black / 0.20 /
+max 200.
+
 # MyMistakes.md
 
 > **Two jobs**, distinguished by heading: