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.
internal/crypto and CompressStream/CompressData no longer exist and are
skipped. The "cut right after the age header and nonce" truncation is
excluded: it reads as valid and empty today and belongs to #152. Reworded
two writer_test.go messages that overstated what the double hash prevents.
Model: opus-4-8
