Close bank-export review findings: name-cap underflow, double overwrite prompt, test scope

Clamps insertSuffix's underflow, floors uniqueEntryName's validity guard, suppresses
the redundant overwrite confirm via a picker out-param, adds a PayloadBuffer
high-water mark, and corrects stale CLAUDE.md/CMake claims.

tests/test_package_io.cpp

@@ -72,7 +72,9 @@ static void testPayloadCounterTracksMovesNotCopies() {
 }
 
 static void testStreamingRoundTripHoldsOnePayload() {
-    const std::string dest = "pkg_io_scratch.rsbank";
+    std::error_code destEc;
+    const std::string dest =
+        (fs::temp_directory_path(destEc) / "pkg_io_scratch.rsbank").generic_string();
     const std::vector<std::uint8_t> header = patternBytes(16, 0xA0);
     const std::vector<std::vector<std::uint8_t>> entries = {
         patternBytes(1000, 1), patternBytes(500, 2), patternBytes(1, 3)};