test(git): seeded differential fuzzer for git-show blob classification

Hermetic temp repo with objects of every relevant type (large/small UTF-8,
Latin-1 and UTF-16 blobs, a tree, commits). A fixed-seed PRNG generates 600
'git show <random flags> <arg>' invocations and checks rtk's classification
(windowed / passthrough / commit-diff) against 'git cat-file -t' ground truth:
0 misroutes (never window a non-blob), 0 silent losses (a large recoverable
UTF-8 blob always windows), and byte-identical passthrough for declined blobs.
A companion test proves the recovery hint reconstructs a windowed blob
byte-for-byte.