Add negative and boundary tests for blobgen and types (closes #170)

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

internal/blobgen/writer_test.go

@@ -13,8 +13,9 @@ import (
 )
 
 // TestWriterHashIsDoubleHash verifies that Writer.ContentID() returns
-// the double hash SHA256(SHA256(plaintext)) for security.
-// Double hashing prevents attackers from confirming existence of known content.
+// SHA256(SHA256(plaintext)). Stored objects are named by this second hash so a
+// name is not the plaintext's own SHA-256; this does not stop someone who
+// already holds the plaintext from confirming it.
 func TestWriterHashIsDoubleHash(t *testing.T) {
 	t.Parallel()
 
@@ -60,11 +61,11 @@ func TestWriterHashIsDoubleHash(t *testing.T) {
 
 	// The writer hash should match the double hash
 	assert.Equal(t, expectedDoubleHash, writerHash,
-		"Writer.ContentID() should return SHA256(SHA256(plaintext)) for security")
+		"Writer.ContentID() must be SHA256(SHA256(plaintext))")
 
-	// Verify it's NOT the single hash (would leak information)
+	// It must be the second hash, not the plaintext's own SHA-256.
 	assert.NotEqual(t, singleHashStr, writerHash,
-		"Writer hash should not be single hash (would allow content confirmation attacks)")
+		"Writer hash must be the double hash, not the single SHA-256")
 }
 
 // TestWriterDeterministicHash verifies that the same input always produces