feat(contract): reject self-claims by the bounty creator

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Description

Enforces an explicit check in submit_claim to reject claims submitted by the bounty creator with error code Error::CreatorCannotClaim (20). This prevents creators from locking funds and later claiming their own bounties.

Closes #171

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Tasks Completed

• Defined CreatorCannotClaim = 20 in Error enum in contracts/deprader_router/src/lib.rs.
• Added if claimant == bounty.creator { return Err(Error::CreatorCannotClaim); } validation to submit_claim.
• Added unit test test_creator_cannot_claim_own_bounty in contracts/deprader_router/src/test.rs.
• Generated test snapshot test_creator_cannot_claim_own_bounty.1.json.
• Documented the change in PROGRESS.md.

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Acceptance Criteria

• Unit test demonstrating the failure: test_creator_cannot_claim_own_bounty
• Error code added to the Error enum: CreatorCannotClaim = 20
• All 22 contract tests and 26 workspace tests pass (cargo test --workspace --locked)
• cargo fmt --all -- --check and cargo clippy --workspace --all-targets --locked -- -D warnings pass cleanly
• cargo build -p deprader-router --target wasm32v1-none --release --locked compiles cleanly
• PROGRESS.md entry added, and the PR description includes Closes #171

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Stellar Address (Stellar Wave Program)

GDLCJGQLXZGDAAT5QGRF6FM2MBEER3FPYGCSVPXYL47G2ABIOX2M2S6K