Add RGB contract consignment import and export

[Jainakin]

Closes #56

Problem

Asset registries need a standard RGB contract consignment that can be verified and imported independently of any asset transfer. An asset ID alone cannot reconstruct or validate the contract, while a transfer consignment carries ownership history that is not appropriate for contract preload.

Changes

• re-export RgbContract and expose Wallet::export_asset_contract
• add offline Wallet::import_asset_contract for contracts without external media
• add Wallet::import_asset_contract_with_media for contracts whose attachment bytes are supplied out of band
• return canonical metadata and whether the complete contract state was already imported
• make repeated imports idempotent and repair media-only, stock-only, and database-only interrupted states
• move pure attachment extraction to the offline wallet surface and cover every NIA, CFA, IFA, and UDA media location
• add explicit RGB stock persistence control so write errors reach the caller and read-only operations do not rewrite stock

The exported value uses rgb-ops' standard ASCII-armored RGB CONSIGNMENT Display/FromStr representation, including the canonical ID, contract ID, schema, version, type, and SHA-256 checksum headers.

Safety and semantics

• import validates the supported schema, exact wallet network, contract identity, and full contract consignment before mutation
• a private non-panicking resolver rejects witness-bearing/history-shaped input
• contract preload registers public contract identity and metadata only; it creates no allocation, transfer, or wallet ownership, so the initial wallet balance is zero
• post-transfer exports are verified to contain only the standalone contract and no private transfer bundles or terminals
• media is matched by the digest committed in the contract, streamed with bounded memory, re-hashed while copying, atomically placed, and directory-synced on Unix targets
• stock is durably persisted before the SQL transaction commits; every interruption prefix is retryable
• the existing save_new_asset transfer-consignment API and its behavior are unchanged

Validation

Green locally:

• cargo fmt -- --check
• strict Clippy with all features, no default features, Electrum-only, and Esplora-only
• release builds with all features, no default features, Electrum-only, and Esplora-only
• no-default-feature tests: 42 passed, including offline export/import
• contract-focused Docker integration tests: 9 passed
• doc tests: 2 passed
• full all-feature suite: 309 passed and 5 ignored

The remaining three local all-feature failures are the existing external multisig-hub tests (multisig::fail, multisig::send_to_oneself, and multisig::success), all returning Missing or invalid credentials from the local hub environment. Upstream Linux CI is still required before this draft is ready for review.

Coverage

Tests cover:

• canonical armor serialization and parser round-trip
• NIA, CFA, IFA, and UDA
• all declared contract/token media locations
• missing, unexpected, duplicate, corrupt, and repaired media
• wrong-network rejection
• malicious history-bearing contract input
• a real export after a settled transfer
• zero-balance semantics and idempotent re-import
• stock-only and database-only recovery
• deterministic runtime-lock contention and successful retry
• unknown-asset export errors

[codecov]

Codecov Report

❌ Patch coverage is 88.18182% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.04%. Comparing base (fd1aca2) to head (a806aee).

Files with missing lines	Patch %	Lines
src/wallet/rust_only.rs	86.64%	37 Missing ⚠️
src/utils.rs	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
- Coverage   96.24%   96.04%   -0.20%     
==========================================
  Files          24       24              
  Lines       13418    13706     +288     
==========================================
+ Hits        12914    13164     +250     
- Misses        504      542      +38     

Flag	Coverage Δ
rust	96.04% <88.18%> (-0.20%)	⬇️

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