Move to the layout REPO_POLICIES.md gives (closes #30)

The Go module moves from backend/ to the root (go.mod, go.sum,
.golangci.yml, cmd/, internal/), the page's source to web/, and nginx's
two files to configs/. backend/ is gone: its LICENSE and .dockerignore
are deleted, and its .gitignore entries, README and scripts are folded
into the root ones. script/build builds the server binary after the
frontend; script/run and script/clean move up from backend/script/;
make test, lint, fmt and fmt-check do what the backend's did. Vite's
root is web/ and it still builds into dist/. go.mod ignores
node_modules/, whose npm packages can ship .go files. script/bootstrap
no longer installs gcc, which only the host Go tests needed.

Model: opus-5-5

internal/reportbuf/export_test.go

@@ -0,0 +1,29 @@
+package reportbuf
+
+import (
+	"os"
+	"time"
+)
+
+// FlushSizeThreshold exposes the buffer size at which Append starts
+// writing a report file to the external tests.
+const FlushSizeThreshold = flushSizeThreshold
+
+// Flush writes the buffered reports to a file now, as the periodic
+// flush does, so tests need not wait a minute for it.
+func (b *Buffer) Flush() error {
+	return b.flushLocked()
+}
+
+// StopClock makes every report file the buffer writes from now on
+// carry the timestamp at, as if all were written in one millisecond.
+func (b *Buffer) StopClock(at time.Time) {
+	b.now = func() time.Time { return at }
+}
+
+// OnFileCreated makes the buffer call fn with each report file it
+// writes from now on, once the file is created and before anything is
+// written to it.
+func (b *Buffer) OnFileCreated(fn func(f *os.File)) {
+	b.fileCreated = fn
+}