perf(updater): read the downloaded archive in one bounded allocation

Before verifying a downloaded archive, both updaters slurped it into a
string with an istreambuf_iterator, which grows the buffer repeatedly
and reallocates as it reads a multi-megabyte file. It also placed no
upper bound on the size it would read into memory.

- A shared readFileExact helper sizes the file first and reads it in a
  single allocation, failing closed if the file is missing, empty,
  larger than a caller-supplied cap, or comes up short. A truncated read
  can never reach the SHA-256 / signature verifier.
- Both updaters use it with the existing max-archive cap.

Tests cover a good read, a read exactly at the cap, over the cap, an
empty file, and a missing file.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/xmrig_updater.cpp

@@ -269,14 +269,8 @@ void XmrigUpdater::installResolved(const std::string& targetDir, const XmrigRele
     setProgress(State::Verifying, "Verifying download…");
     std::string bytes;   // kept in scope through extraction so we extract the VERIFIED buffer (I-01)
     {
-        std::ifstream f(zipPath, std::ios::binary);
-        if (!f) {
-            fs::remove(zipPath, ec);
-            setProgress(State::Failed, "Could not read the downloaded archive.");
-            return;
-        }
-        bytes.assign(std::istreambuf_iterator<char>(f), std::istreambuf_iterator<char>());
-        if (f.bad()) {
+        if (!readFileExact(zipPath, bytes,
+                           static_cast<std::uintmax_t>(kMaxArchiveBytes))) {
             fs::remove(zipPath, ec);
             setProgress(State::Failed, "Could not read the downloaded archive.");
             return;