Seed from go-template-repo, renamed to simplexcalc

The template's files at a77fd30, without its history or LICENSE, after
script/rename simplexcalc.

Model: opus-5-5

internal/render/errors.go

@@ -0,0 +1,13 @@
+package render
+
+import "errors"
+
+var (
+	// errNoTemplates means the embed matched nothing — a build that
+	// produced a binary with no pages in it.
+	errNoTemplates = errors.New("no page templates were embedded")
+
+	// errUnknownTemplate means a handler asked for a page that is not
+	// in the embedded set: a typo, caught by that handler's test.
+	errUnknownTemplate = errors.New("unknown template")
+)