refactor: rename inspect to replay across the codebase

Renames inspect-ui/ to replay-ui/, internal/inspect/ to internal/replay/,
the CLI subcommand from `sanderling inspect` to `sanderling replay`, and
updates all references in docs, Makefile, README, and Go comments.

internal/replay/assets.go

@@ -0,0 +1,21 @@
+// Package replay serves the embedded web UI for browsing recorded runs.
+package replay
+
+import (
+	"embed"
+	"io/fs"
+)
+
+//go:embed all:dist
+var distFS embed.FS
+
+// Assets returns the embedded SPA bundle rooted at the dist directory.
+// In Stage 2 this contains a stub index.html. Stage 4 wires the real
+// bundle in via Makefile (copy replay-ui/dist -> internal/replay/dist).
+func Assets() fs.FS {
+	sub, err := fs.Sub(distFS, "dist")
+	if err != nil {
+		panic("replay: dist embed missing: " + err.Error())
+	}
+	return sub
+}