feat: add proxy contract for atomic batch mint_wrap

[ChukwuebukaJohnDev]

Overview

This PR adds a Soroban proxy contract that atomically batches multiple mint_wrap calls against the Stellar asset wrapping contract. The proxy validates the caller, forwards each mint instruction sequentially within a single transaction, and reverts the entire batch if any call fails — enabling efficient multi-asset wrapping without partial state changes.

Related Issue

Changes

⚙️ Proxy Contract

• [ADD] proxy/Cargo.toml

  • Soroban SDK dependencies and panic/error handling.
  • Optimized release profile for WASM size and security audits.

• [ADD] proxy/src/lib.rs

  • batch_mint_wrap entrypoint accepting a vector of (asset, amount, recipient) mint orders.
  • Per-order mint_wrap invocation via the Soroban host environment.
  • Atomicity guard: any failed sub-call triggers a full rollback of the batch.
  • Access-control check to restrict batching to authorized callers.
  • Emits a single BatchMintWrap event with per-order results.

• [ADD] proxy/src/test.rs

  • Unit tests for empty batches, successful multi-mint batches, single-mint failure rollback, and unauthorized caller rejection.
  • Edge cases: duplicate assets, zero amounts, and max batch size.

📚 Documentation & CI

• [ADD] PROXY_PATTERN_DECISION.md

  • Explains the proxy architecture decision: direct mint_wrap forwarding vs. token contract mint via admin.
  • Details atomicity guarantees, reentrancy considerations, and upgrade path.

• [MODIFY] .github/wasm-size-limit

  • Set a WASM size budget for the new proxy contract to keep CI size audits passing.

Verification Results

cargo test -p proxy
✅ 14/14 passed

cargo clippy -- -D warnings
✅ 0 warnings

cargo wasm-size-limit proxy 32KB
✅ proxy.wasm size: 28.4 KB (under limit)

Live Stellar testnet check:
✅ Batch of 5 mint_wrap calls applied atomically
✅ Forced failure in item 3 rolled back all previous mints
✅ Unauthorized caller rejected before any mint

Acceptance Criteria	Status
Proxy contract batches multiple mint_wrap calls atomically	✅ All sub-calls committed or reverted as a unit
Comprehensive tests cover all edge cases	✅ 14 tests covering success, rollback, auth, empty, zero, duplicate, and max-size cases
Code compiles cleanly and passes security/size audits	✅ clippy clean, 28.4 KB WASM under CI size limit
Documentation for architectural changes provided	✅ PROXY_PATTERN_DECISION.md added

Closes #517