test-tool synthesize: add precomputed SHA-256 pack for 4 GiB + 1

test-tool synthesize: add precomputed SHA-256 pack for 4 GiB + 1

Add a SHA-256 entry to the fast_packs[] table. The pack prefix and
deflate block structure are identical to SHA-1 (the pack format does
not encode the hash algorithm in its header). Only the suffix differs:
SHA-256 OIDs are 32 bytes instead of 20, giving a 609-byte suffix
compared to 513 for SHA-1, and a different pack checksum.

The constants were generated by running the generic path inside a
repository initialized with --object-format=sha256.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>