docs(package): record known gaps and correct stale claims

Notes the export verb's overwrite-consent obligation post-append, the append's
extension-divergence behavior, and readFilePayload's 4GiB blind spot; fixes a
stale u8string() reference and marks the 4GiB guard as accepted-unexercised.

src/shell/package/package_io.h

@@ -101,7 +101,9 @@ private:
 };
 
 // One source file read whole as one entry's payload — a bank file IS the streaming
-// unit. Empty on any failure, per PackageFileReader.
+// unit. Empty on any failure, per PackageFileReader — including a source file over
+// readRange's 4 GiB ceiling, which reads as empty exactly like an unreadable file;
+// fileStatus() cannot tell the two apart either, since it only checks openability.
 PayloadBuffer readFilePayload(const std::string& absPath);
 
 // Export must tell a missing indexed file from an unreadable one in its refusal