Test-only. internal/blobgen and internal/types had no negative or boundary coverage. Adds, in package blobgen_test: Writer-to-Reader round trips at the 64 KiB age-segment edges for random and compressible data, checking plaintext, byte counts and the reader/writer hashes by decrypting; a wrong-identity open; truncation and single-byte corruption of a multi-segment blob at every region; trailing bytes, empty input and garbage; rejected and accepted compression levels; nil, empty and invalid recipients; and a failing destination. In package types_test: Value/Scan round trips, NULL, wrong-type and malformed Scan, Parse and IsZero for FileID and BlobID. The "cut right after the age header and nonce" truncation is excluded: it reads as valid and empty today and belongs to #152. Model: opus-4-8