ddk: own the BIP-329 contract label text in contract::labels

[bennyhodl]

Summary

Item 9 of #225. DDK's wallet sync writes DLC funding <id>, the funding outpoint label and DLC close <id>: <outcomes> inline, and the Lygos wallet writes the same strings on its own. ddk::contract::labels now owns that text, so every DDK wallet's BIP-329 export reads the same. It builds without the manager feature.

Changes

• Add contract::labels:
  • funding_labels(contract_id, outpoint) -> FundingLabels returns the transaction label and the output label, each paired with the txid or outpoint it belongs on.
  • close_label(contract_id, outcomes) -> String.
  • Both return plain strings and use no bip329 types, so a stateless wallet wraps them in its own record type.
• wallet/command.rs uses the module for the funding labels and the close label. spendable: Some(false) stays with the BIP-329 record it belongs to.
• The text is unchanged. One test pins it, including outcomes that join to an empty string.

Not changed

• Lygos also writes DLC refund <id> and DLC cancel <temp id>. DDK has no refund or cancel label today; a refund close is written as DLC close <id>. Adding them would change exports on purpose, so it's left for a separate decision.

Testing

• cargo test -p ddk --lib: 81 passed, including the new label_text_is_stable and the existing label tests.
• cargo test -p ddk --test enumeration -- --ignored: the end-to-end flow writes the funding, outpoint and close labels and round-trips the JSONL export.
• cargo fmt --check, cargo clippy -- -D warnings and cargo check -p ddk --no-default-features pass.
• Reviewed by GPT-6 Astra. It found one regression: a single empty outcome gained a trailing : . That is fixed, and the test covers it.