feat(stellar): Add stealth-batch-sender Soroban contract (#47)

[cokehill]

Summary

Closes #47

Implements stealth-batch-sender — a new Soroban crate that sends tokens
to N pre-computed stealth addresses atomically in one transaction, matching
the EVM WraithSender.batchSendETH / batchSendERC20 functionality.

What's in this PR

• New crate: contracts/stellar/stealth-batch-sender/
• batch_send(from, transfers: Vec<Transfer>, asset) entrypoint
  • Transfer carries stealth_address, ephemeral_pub_key, amount
  • Single from.require_auth() covers the entire batch
• All-or-nothing atomicity via Soroban's transaction model
• Per-transfer ANNOUNCE event + batch-level BATCH summary event
• MAX_BATCH_SIZE = 100 (justified: ~50M instructions vs ~100M budget)
• max_batch_size() read-only query for SDK/UI consumers

Tests

• ✅ Success: 3-transfer batch, verifies balances
• ✅ Atomicity: mid-batch failure panics and rolls back all transfers
• ✅ Max size cap: 101 transfers → panic
• ✅ Empty batch rejected
• ✅ Zero-amount transfer rejected
• ✅ max_batch_size() returns 100

Resource budget comparison vs N individual stealth-sender calls

Approach	Auth ops	Ledger entries	Approx instructions
N × stealth-sender::send	N	N × 2	N × ~500K
batch_send (N=100)	1	1 + N × 1	~52M

Batch is significantly more efficient due to single auth and single
transaction envelope.

Follow-up issues to file (as noted in acceptance criteria)

• SDK: buildBatchSendStellar builder function
• Demo: batch-send UI screen

[drips-wave]

@cokehill Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[truthixify]

Just one conflict in stellar/Cargo.toml (workspace members list). Quick rebase:

git fetch origin
git rebase origin/develop
git push --force-with-lease

Keep both: develop's existing members plus your new stealth-batch-sender entry. The crate files themselves merge cleanly.

[cokehill]

resolved

[truthixify]

Clean merge after rebase. New stealth-batch-sender crate gives Stellar atomic batch send parity with the EVM WraithSender.batchSendETH/batchSendERC20. Workspace wiring and tests look good. Thanks @cokehill.