feat(stellar): integrate on-chain metadata resolver for wraith-names

[OTimileyin]

Closes #119

Summary

Add optional on-chain metadata resolver for wraith-names, allowing name
owners to attach text records and content hashes to their .wraith names,
similar to ENS text records. The feature is opt-in, owner-gated, and avoids
the resolution hot path.

Changes

New: src/metadata.rs

• MetadataEntry struct with text_records: Map<String, String> and
  content_hash: BytesN<32>
• MetadataError enum with 5 dedicated error codes (101–105):
  MetadataKeyTooLong, MetadataValueTooLong, MetadataRecordTooLong,
  MetadataTotalTooLong, MetadataNotFound
• Validation functions with per-record (key ≤ 64B, value ≤ 256B) and total
  (≤ 1024B) size limits
• Unit tests for validation logic

Modified: src/lib.rs

• DataKey::Metadata(BytesN<32>) storage key variant for per-name metadata
• 5 new NamesError variants (50–54) for metadata error propagation
• set_metadata(env, owner, name, metadata) — owner-gated setter with full
  validation
• get_metadata(env, name) — public reader, returns NameNotFound or
  MetadataNotFound
• Metadata cleanup on release() — metadata is removed when a name is
  released
• MetadataUpdated event emission on metadata changes

Hot path untouched

• resolve() and name_of() remain completely unchanged — no metadata
  lookups added to the resolution path

Tests

• test_set_and_get_metadata — happy path with avatar + twitter records
• test_set_metadata_not_owner — non-owner rejected with NotOwner
• test_set_metadata_key_too_long — 65B key → MetadataKeyTooLong
• test_set_metadata_value_too_long — 257B value → MetadataValueTooLong
• test_set_metadata_total_too_long — 5 records × 210B → MetadataTotalTooLong
• test_get_metadata_not_found — name exists, no metadata → MetadataNotFound
• test_get_metadata_unregistered_name — name doesn't exist → NameNotFound
• test_metadata_cleaned_on_release — metadata removed when name is released
• test_hot_path_unchanged_after_metadata — resolve() and name_of() work
  identically before/after metadata
• 3 existing metadata.rs module tests for isolated validation

All 17 wraith-names tests pass, cargo fmt clean, zero warnings.