feat: add network-aware Stellar asset registry and validation layer (Closes #23)

[deepak0x]

Description

Implements a network-aware Stellar asset registry and validation layer (issue #23) on top of the existing assets.ts primitives and shared StellarAssetSchema.

What changed

• packages/types/src/index.ts — added the ASSET_UNSUPPORTED StellarErrorCode so unsupported assets get a typed, user-safe error.
• packages/stellar-kit/src/assetRegistry.ts (new):
  • AssetSupport (supported | testnetOnly | unsupported) and RegistryEntry/AssetRegistry types.
  • createAssetRegistry(entries) — builds a keyed registry (native XLM always supported implicitly).
  • lookupAsset(asset, network, registry?) — returns the support state + typed ASSET_UNSUPPORTED error; never throws.
  • validateAssetOnNetwork(asset, network, registry?) — structure + network support; throws ASSET_INVALID / ASSET_UNSUPPORTED.
  • checkAssetOnNetwork(asset, network, registry?) — safe, no-throw variant.
  • DEFAULT_TESTNET_REGISTRY — MVP defaulting to safe testnet examples (native XLM + a demo testnet USDC).
• packages/stellar-kit/src/index.ts — exports the registry module.
• packages/stellar-kit/test/assetRegistry.test.ts (new) — 11 tests: native always supported, testnet asset on testnet, testnet-only on mainnet → testnetOnly, unregistered → unsupported, invalid structure → ASSET_INVALID, safe checkAssetOnNetwork, and custom mainnet registry.
• examples/assets-registry.testnet.json — fixture showing the testnet registry shape.
• apps/web/app/assets/page.tsx (new) — UI: check any asset string against a selected network, see the support state / ASSET_UNSUPPORTED message, and list registered assets.
• docs/asset-registry.md — registry reference + configuration guidance.

Verification

• packages/stellar-kit full suite: 138 tests passing (11 new for the registry).
• packages/stellar-kit builds; web app typechecks clean.
• Native + issued assets represented; network-specific support handled; unsupported assets return typed errors; test/UI/docs included.

Acceptance criteria

• Native and issued assets are represented.
• Network-specific asset support is handled (testnet-only → testnetOnly).
• Unsupported assets return typed errors (ASSET_UNSUPPORTED).
• Payment/anchor utilities can use the shared asset layer (registry sits on the shared StellarAssetSchema/assets.ts primitives they already consume).
• Tests cover native, issued, invalid, and unsupported assets.
• Documentation explains asset configuration.
• MVP defaults to safe testnet examples.

Notes

• The MVP ships only testnet demo assets; production mainnet lists are supplied via a custom registry (documented).
• No secrets involved — issuers are public Stellar accounts (generated testnet key for the demo USDC).