feat : cargo-fuzz harness for stealth-batch-sender

[Olowodarey]

Fuzz harness (stellar/stealth-sender/fuzz/):

• batch_decode — round-trips arbitrary (often mismatched-length) batches through the same XDR encoding a real contract invocation uses, asserting no drift and no panics. Pure decode-path fuzzing, no contract calls.
• batch_execute — deploys a real StealthAnnouncer + SAC token, calls batch_send with arbitrary lengths/duplicate recipients/negative-zero-large amounts via try_batch_send, and asserts: success implies matched lengths, exact balance movement per recipient (no overwrites), and event count == batch length; failure implies balances/events are byte-for-byte unchanged (atomicity holds).
• Both ran clean locally (76k and 7.3k execs/60s, zero crashes), and a minimized seed corpus (217 files, ~888KB) is committed under fuzz/corpus/.

closes #109

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[truthixify]

Merged. Fuzz harness for stealth-batch-sender is a solid safety addition, thanks.