Add sponsored announcements to Stellar sender

[OlyraCreations]

Summary

Adds the fee-payer/sponsored announcement pattern requested in #101. Platforms can submit multiple independently authenticated announcement entries in one Soroban invocation while a sponsor supplies the transaction fee through Stellar fee-bump mechanics.

Fixes #101

What changed

• Added SponsoredEntry to stellar/stealth-sender/src/lib.rs with the per-entry sender, token, amount, scheme ID, stealth address, ephemeral public key, and metadata.
• Added sponsored_announce(sponsor, entries).
• Requires the sponsor to authorize the invocation and requires every entry sender to authorize its own transfer and announcement.
• Preserves the existing asset-policy checks, protocol-fee handling, token transfers, announcements, TTL extension, and sender metrics.
• Added SponsoredBatchTooLarge = 6 and enforced a maximum of 20 entries.
• Added successful and negative unit tests for sponsored announcements.
• Added stellar/integration-tests/sponsored.rs to verify a submitted futurenet transaction records the sponsor as Horizon's fee_account.
• Added sponsored batch cases to stellar/bench/src/main.rs.
• Recorded measured Soroban resource usage for 1, 5, 10, and 20 entry batches in stellar/PERF.md.
• Regenerated the Stellar sender TypeScript binding with the new method, entry type, and error.

Acceptance criteria

• Optimized sender WASM remains below 110 KB.
  • Measured optimized size: 15,732 bytes.
• Size cap is enforced with a dedicated error type.
  • 21 entries return SenderError::SponsoredBatchTooLarge.
• Per-entry resource cost is recorded in PERF.md.
• Sender bindings were regenerated and verified with no binding drift.
• Futurenet fee-source verification test added.
  • The test is ignored by default and runs with FUTURENET_TX_HASH and FUTURENET_SPONSOR_ADDRESS against Horizon after a fee-bumped transaction is submitted.

Validation

• cargo test -p stealth-sender: passed.
  • Sender unit, fee, property, SAC compatibility, and upgrade-authority tests passed.
• Optimized WASM build and size check passed: 15,732 bytes.
• Binding regeneration produced no diff.
• New integration test passed Rust formatting/syntax validation.

Known repository issue

The full cargo test --workspace check is currently blocked by pre-existing unrelated errors in stealth-registry/src/lib.rs (mismatched closing delimiter). Workspace-wide formatting also reports existing formatting differences outside the files changed by this PR.

The pre-existing untracked root package-lock.json was not included in this PR.

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[truthixify]

Rebase on latest develop and force-push. Your branch was pushed before the CI trigger was updated to run on develop PRs, so CI hasn't validated it yet.

[OlyraCreations]

Thanks so much for the feed back

[truthixify]

Merge conflicts against develop after the merge cascade. Rebase and CI should look different against the fixed workspace.