fix: HEAD requests fetch and stream the full body (#89)

Fixes #70

## Why
Docker `HEAD` routes mapped to `handleProxy`, which ran a full `Fetch` + `io.Copy` — downloading the entire blob (and fetching upstream on a miss) only for net/http to discard the body. HEAD existence checks (manifests, blobs) are common.

## Changes
- Add `Engine.Head`: answers cached artifacts/indexes from store metadata (no blob download); on a miss issues an upstream `HEAD` (with bearer-token handling) and never caches a body.
- Route `HEAD /v2/{remote}/*` to a dedicated `handleProxyHead` that writes headers only.
- Add e2e tests for HEAD on a blocklisted path (403) and an unknown remote (404).

## Note
`headUpstream` uses `http.DefaultClient` to build cleanly on master; it will pick up the shared timeout-configured client from #67 once that merges.

## Validation
- `make e2e` passes (includes new HEAD tests).

Reviewed-on: #89
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

e2e/proxy_test.go

@@ -24,6 +24,39 @@ func TestProxyBlocklist(t *testing.T) {
 	assertStatus(t, apiURL("/api/v1/remote/blocklist-test/malware.exe"), http.StatusForbidden)
 }
 
+func TestProxyHeadBlocklist(t *testing.T) {
+	createRemote(t, `{
+		"name": "head-block-test",
+		"package_type": "generic",
+		"base_url": "https://example.com",
+		"blocklist": ["\\.exe$"],
+		"stale_on_error": true
+	}`)
+	defer deleteRemote(t, "head-block-test")
+
+	req, _ := http.NewRequest(http.MethodHead, apiURL("/v2/head-block-test/malware.exe"), nil)
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("HEAD: %v", err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("HEAD blocklisted path: got %d, want 403", resp.StatusCode)
+	}
+}
+
+func TestProxyHeadUnknownRemote(t *testing.T) {
+	req, _ := http.NewRequest(http.MethodHead, apiURL("/v2/nonexistent/some/path"), nil)
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("HEAD: %v", err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusNotFound {
+		t.Fatalf("HEAD unknown remote: got %d, want 404", resp.StatusCode)
+	}
+}
+
 func TestProxyPatterns(t *testing.T) {
 	createRemote(t, `{
 		"name": "patterns-test",