Updates: when an answer is the rest of the same file, and how long to try

For carrying on a download that broke (issue #54): the head's ETag and
Content-Range are read, a 206 is checked to be the rest of the same
file, and a policy says how many tries a download is worth. Host-tested.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0162FokPdvY2KsS4NBfwyWPk

lib/release/src/http_head.cpp

@@ -1,5 +1,7 @@
 #include "http_head.h"
 
+#include <cstdio>
+
 #include <algorithm>
 #include <cctype>
 #include <cstdlib>
@@ -58,6 +60,15 @@ void HttpHeadParser::line() {
     else if (name == "transfer-encoding") head_.chunked = lower(value).find("chunked") != std::string::npos;
     else if (name == "location") head_.location = value;
     else if (name == "content-type") head_.contentType = value;
+    else if (name == "etag") head_.etag = value;
+    else if (name == "content-range") {
+        long from = -1, to = -1, total = -1;
+        if (std::sscanf(lower(value).c_str(), "bytes %ld-%ld/%ld", &from, &to, &total) == 3 && from >= 0 && to >= from && total > to) {
+            head_.rangeFrom = from;
+            head_.rangeTo = to;
+            head_.rangeTotal = total;
+        }
+    }
 }
 
 size_t ChunkedDecoder::decode(const uint8_t* in, size_t len, uint8_t* out) {