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/server/staticfs.go

@@ -0,0 +1,48 @@
+package server
+
+import (
+	"io/fs"
+	"net/http"
+)
+
+// filesOnly wraps an http.FileSystem so that directories do not exist as
+// far as http.FileServer is concerned. FileServer asks the filesystem for
+// the directory first and generates its index from what it gets back;
+// refusing the Open is therefore the whole control, and it leaves the
+// path handling, content sniffing and range support of FileServer intact.
+//
+// Enumerating what a binary ships is a capability no caller needs, and
+// the embedded set grows as a project seeded from this template adds to
+// it.
+type filesOnly struct {
+	inner http.FileSystem
+}
+
+// Open serves a file and refuses a directory with fs.ErrNotExist, which
+// http.FileServer maps to 404 — the same answer a path that is not
+// embedded at all gets.
+func (f filesOnly) Open(name string) (http.File, error) {
+	file, err := f.inner.Open(name)
+	if err != nil {
+		// Unwrapped on purpose: http.FileServer inspects this error,
+		// and wrapping hides fs.ErrNotExist from it.
+		//nolint:wrapcheck // see above.
+		return nil, err
+	}
+
+	info, err := file.Stat()
+	if err != nil {
+		_ = file.Close()
+
+		//nolint:wrapcheck // as above.
+		return nil, err
+	}
+
+	if info.IsDir() {
+		_ = file.Close()
+
+		return nil, fs.ErrNotExist
+	}
+
+	return file, nil
+}