feat: add maintainer-approved bounty claims

[TUPM96]

Summary

• add BountyEscrow storage that binds each issue bounty to the token and amount actually locked
• implement maintainer-approved claim_bounty(maintainer, issue_id, contributor) to release escrowed tokens to the contributor
• clear escrow state before payout so the same bounty cannot be claimed twice
• update cancel_bounty to return the stored escrow instead of accepting arbitrary token/amount inputs
• add token-backed Soroban tests for create, claim, token binding, cancel, missing escrow, and double-claim prevention

Validation

• cargo fmt
• cargo test (6 passing)
• cargo clippy --all-targets -- -D warnings
• cargo build --target wasm32v1-none --release
• git diff --check

Closes #2

Payout details can be provided privately if needed.

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