fix(nntp): support open-ended OVER/XOVER ranges ("n-")

parseRange treated a range of the form "low-" (low article to the
highest available) as malformed and returned an empty (0,0) range, so
OVER/XOVER "n-" returned no articles instead of every article >= n.
RFC 3977 defines a range as a single number, "number-", or
"number-number"; the open-ended form is common in real NNTP clients.

Handle "low-" as (low, MaxInt64), reject specs with more than one dash
(e.g. "1-2-3") as malformed, and keep existing malformed handling so a
bad range still yields an empty range rather than "all articles".
Adds tests for closed, open-ended, empty, and malformed specs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/news/nntpd/server.go

@@ -177,10 +177,19 @@ func parseRange(spec string) (low, high int64) {
 		}
 		return h, h
 	}
+	if len(parts) != 2 {
+		return 0, 0 // malformed (e.g. "1-2-3") — empty range
+	}
 	l, err := strconv.ParseInt(parts[0], 10, 64)
 	if err != nil {
 		return 0, 0 // malformed — empty range
 	}
+	// Open-ended range "low-" means "from low to the highest article"
+	// (RFC 3977: a range is a single number, "number-", or
+	// "number-number"). An empty upper bound is unbounded, not malformed.
+	if parts[1] == "" {
+		return l, math.MaxInt64
+	}
 	h, err := strconv.ParseInt(parts[1], 10, 64)
 	if err != nil {
 		return 0, 0 // malformed — empty range