initial scaffold

- module layout: cmd/mithril-go, internal/{aggregator,artifact,verify,networks}
- aggregator REST client, list command working against mainnet
- download/extract/verify stubbed
- no deps yet, pure stdlib

internal/artifact/download.go

@@ -0,0 +1,27 @@
+// Package artifact handles downloading and extracting Mithril snapshot artifacts.
+// Currently stubs — HTTP range requests, resumable downloads, zstd+tar extraction
+// will be implemented in the next pass.
+package artifact
+
+import (
+	"context"
+	"errors"
+)
+
+var ErrNotImplemented = errors.New("not yet implemented")
+
+// Download fetches an artifact from one of the supplied locations, choosing
+// the first reachable one and storing it at destPath.
+// Implementation will do:
+//   - parallel range-chunks over HTTP
+//   - resume on partial .part file
+//   - SHA-256 verification against the snapshot manifest
+func Download(ctx context.Context, locations []string, destPath string) error {
+	return ErrNotImplemented
+}
+
+// Extract decompresses a zstd+tar archive into targetDir.
+// Will stream through zstd -> tar reader without buffering the full archive.
+func Extract(ctx context.Context, archivePath, targetDir string) error {
+	return ErrNotImplemented
+}