Show mod downloads and release dates in the launcher MODS panel

The MODS tab now shows each installed mod's total GitHub downloads
(summed asset download_count across all releases), its first and latest
release dates, and a Sort row (Name / Popularity / Release date /
Last updated) persisted in options.modSort.

The launcher already fetched per-repo release lists for update checks, so
the data rides the same cached fetch: parseRelease keeps download_count
and published_at, writeCache persists them, and a cache entry written
before the fields existed is treated as stale and refetched once instead
of hiding the line behind an old cache.

tests/engine/mod_update_tests.lua

@@ -108,4 +108,133 @@ do
   check(preview:find("Changes", 1, true), "previewLine keeps heading text")
 end
 
+-- downloads: per-release sum over every asset's download_count
+do
+  local body = Json.encode({
+    {
+      tag_name = "v1.2.0",
+      assets = {
+        { name = "demo-1.2.0.zip", browser_download_url = "https://x/d.zip",
+          size = 99, download_count = 41 },
+        { name = "demo-1.2.0.sha256", browser_download_url = "https://x/d.sha",
+          download_count = 9 },
+      },
+    },
+  })
+  local list = ModUpdate.parseReleases(body, "demo")
+  eq(list[1].downloads, 50, "downloads sums every asset, not just the zip")
+end
+
+-- published: the release date is kept as an ISO day
+do
+  local body = Json.encode({
+    { tag_name = "v1.2.0", published_at = "2025-04-13T09:24:00Z",
+      assets = { { name = "demo-1.2.0.zip",
+        browser_download_url = "https://x/d.zip" } } },
+  })
+  local list = ModUpdate.parseReleases(body, "demo")
+  eq(list[1].published, "2025-04-13", "published_at is reduced to the day")
+  local noDate = ModUpdate.parseReleases(Json.encode({
+    { tag_name = "v1.0.0",
+      assets = { { name = "demo-1.0.0.zip",
+        browser_download_url = "https://x/d.zip" } } },
+  }), "demo")
+  check(noDate[1].published == nil, "missing dates stay nil, never throw")
+end
+
+-- releaseDates: first and latest across releases
+do
+  local d = ModUpdate.releaseDates({
+    { version = "1.0.0", published = "2024-05-31" },
+    { version = "1.2.0", published = "2025-11-02" },
+    { version = "1.1.0", published = "2025-01-15" },
+  })
+  eq(d.first, "2024-05-31", "first is the oldest release date")
+  eq(d.latest, "2025-11-02", "latest is the newest release date")
+  check(ModUpdate.releaseDates({ { version = "1.0.0" } }) == nil,
+    "releases without dates report no data")
+  check(ModUpdate.releaseDates({}) == nil, "empty list reports no data")
+  check(ModUpdate.releaseDates(nil) == nil, "nil list reports no data")
+end
+
+-- totalDownloads: totals across releases; nil until data actually exists
+do
+  local new = {
+    { version = "1.0.0", downloads = 41 },
+    { version = "1.1.0", downloads = 9 },
+  }
+  local dl = ModUpdate.totalDownloads(new)
+  eq(dl.total, 50, "totals across releases")
+  eq(dl.releases, 2, "counts the releases")
+  dl = ModUpdate.totalDownloads({ { version = "1.0.0", downloads = 0 } })
+  eq(dl.total, 0, "a real zero stays zero")
+  dl = ModUpdate.totalDownloads({ { version = "1.0.0" } })
+  check(dl == nil, "pre-downloads cache rows report no data")
+  check(ModUpdate.totalDownloads({}) == nil, "empty list reports no data")
+  check(ModUpdate.totalDownloads(nil) == nil, "nil list reports no data")
+end
+
+-- formatCount: thousands separators, never throws
+eq(ModUpdate.formatCount(0), "0", "zero formats plain")
+eq(ModUpdate.formatCount(999), "999", "below 1000 formats plain")
+eq(ModUpdate.formatCount(1000), "1,000", "1000 gets a separator")
+eq(ModUpdate.formatCount(1234567), "1,234,567", "large counts group by 3")
+eq(ModUpdate.formatCount("12345"), "12,345", "numeric strings are accepted")
+eq(ModUpdate.formatCount(nil), "0", "nil formats as zero")
+eq(ModUpdate.formatCount("garbage"), "0", "garbage formats as zero")
+
+-- cacheUsable: a cache entry from before downloads existed must not be
+-- trusted, everything current is
+do
+  local old = { checkedAt = os.time(),
+    releases = { { version = "1.0.0", tag = "v1.0.0" } } }
+  check(not ModUpdate.cacheUsable(old), "pre-downloads cache is unusable")
+  local zero = { checkedAt = os.time(),
+    releases = { { version = "1.0.0", downloads = 0 } } }
+  check(ModUpdate.cacheUsable(zero), "current cache is usable even at zero")
+  check(ModUpdate.cacheUsable({ checkedAt = os.time(), releases = {} }),
+    "empty release list stays usable")
+  check(not ModUpdate.cacheUsable(nil), "nil cache is unusable")
+  check(not ModUpdate.cacheUsable({}), "cache without releases is unusable")
+end
+
+-- fetchReleases: an old-format cache entry is refetched once and rewritten
+-- in the current format; a current one is served untouched
+do
+  local HostShell = require("src.core.HostShell")
+  local realRead, realWrite = ModUpdate.readCache, ModUpdate.writeCache
+  local realCanFetch, realHttpGet = HostShell.canFetch, HostShell.httpGet
+  local cached, fetched
+  ModUpdate.readCache = function() return cached end
+  ModUpdate.writeCache = function(repo, releases)
+    fetched = releases
+    return true
+  end
+  HostShell.canFetch = function() return true end
+  HostShell.httpGet = function()
+    return Json.encode({ {
+      tag_name = "v1.0.0",
+      assets = { { name = "demo-1.0.0.zip",
+        browser_download_url = "https://x/d.zip", download_count = 7 } },
+    } })
+  end
+
+  cached = { checkedAt = os.time(),
+    releases = { { version = "1.0.0", tag = "v1.0.0" } } }
+  fetched = nil
+  local list = ModUpdate.fetchReleases("acme/mod", "demo")
+  check(fetched ~= nil, "old-format cache triggers a refetch")
+  check(list[1].downloads == 7, "refetched release carries downloads")
+
+  cached = { checkedAt = os.time(),
+    releases = { { version = "1.0.0", tag = "v1.0.0", downloads = 0 } } }
+  fetched = nil
+  list = ModUpdate.fetchReleases("acme/mod", "demo")
+  check(fetched == nil, "current cache is served without refetch")
+  check(list[1].downloads == 0, "cached zero stays zero")
+
+  ModUpdate.readCache, ModUpdate.writeCache = realRead, realWrite
+  HostShell.canFetch, HostShell.httpGet = realCanFetch, realHttpGet
+end
+
 print("ok mod_update_tests")