feat: implement issues #83, #89, #98, #108

[leojay-net]

feat: implement issues #83, #89, #98, #108

#83 — StableSwap Invariant AMM (Curve Stableswap Math)

• Add contracts/stableswap/ Soroban contract implementing the Stableswap
  invariant (Curve-style, n=2 tokens) using Newton-Raphson iteration for D
• Implement initialize, add_liquidity, remove_liquidity, and swap functions
• Dynamic fee adjustment: fee = base_fee + imbalance_ratio * fee_multiplier
  where imbalance_ratio = |x-y|/(x+y), increasing fee on imbalanced pools
• Add compute_d and compute_y Newton-Raphson math helpers with overflow guards
• 11 unit tests covering invariant correctness, symmetry, edge cases, fee logic,
  and initialization validation
• Register in contracts Cargo.toml workspace

#89 — Stealth Addresses for Private Yield Deposits

• Add contracts/stealth_addresses/ Soroban contract implementing a
  stealth address registry and deposit routing
• StealthMetaAddress: recipients publish view + spend public keys (32-byte
  Ed25519 keys) via register_meta_address
• deposit: sender provides pre-computed one_time_address (derived off-chain via
  DH: s=H(rKv), P=sG+Kse), ephemeral_public_key R, and view_tag byte;
  funds held in contract escrow
• claim_deposit: only holder of one_time_address private key can claim funds,
  enforced by require_auth
• 8 unit tests covering registration, key validation, deposit validation, and
  claim error handling
• Register in contracts Cargo.toml workspace

#98 — Automated Slither/Mythril Security Pipeline

• Add .github/workflows/security.yml triggered on PR/push to main when
  contracts/ paths change
• cargo-audit job: scans dependencies for known CVEs, parses JSON results,
  posts a formatted vulnerability table as a PR comment (upsert pattern)
• clippy-security job: runs Clippy with -D clippy::unwrap_used,
  -D clippy::expect_used, -D clippy::panic, -D clippy::arithmetic_side_effects,
  -D clippy::indexing_slicing; posts lint violations table as PR comment
• soroban-unsafe-patterns job: grep-based scan for unsafe blocks, use std::
  in no_std contracts, and explicit panic! calls; posts findings as PR comment
• All jobs upload artefacts and always-post PR comments (upsert to avoid spam)

#108 — Unified Multi-Wallet Connector Library

• Add client/src/auth/walletAdapters.ts: WalletAdapter interface with
  isAvailable, getPublicKey, signTransaction; implementations for Freighter
  (@stellar/freighter-api), xBull (@creit.tech/xbull-wallet-connect), Albedo
  (@albedo-link/intent); EXTENSION_ADAPTERS registry + getAdapter()
• Extend WalletProviderId to include 'xbull' | 'albedo' (new
  ExtensionWalletProviderId union type)
• Update session.ts: unified extension-wallet branch routes through adapter,
  removing per-wallet Freighter-specific SDK calls
• Update WalletContextObject.ts: add signTransaction(xdr, passphrase) to
  WalletContextValue interface
• Update WalletContext.tsx: implement signTransaction using the active
  provider's adapter; expose via context value
• Update soroban.ts: executeContractCall, deposit, withdraw accept optional
  signTx parameter; callers can pass wallet.signTransaction from useWallet()
  so any wallet adapter works transparently
• Update WalletConnectionModal.tsx: add xBull and Albedo buttons in new
  'Browser Wallets' section alongside Freighter
• Install @creit.tech/xbull-wallet-connect@0.4.0 and @albedo-link/intent@0.13.0

Closes #83
Closes #89
Closes #98
Closes #108

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