Vault: restore clean `lib.rs` build (merge artifact removal)

[greatest0fallt1me]

Description

The vault crate must compile cleanly on stable Rust with soroban-sdk 22; remove merge markers, duplicate impl blocks, and inconsistent function signatures.

Requirements and context

• This is blocking for CI, coverage, and audits.
• Reconcile the intended public API with README.md.

Suggested execution

• Fork the repo and create a branch: git checkout -b fix/vault-lib-merge
• Implement changes in the Callora-Contracts Soroban workspace (contracts/*/src/).
• Run cargo fmt, cargo clippy --all-targets --all-features -- -D warnings, and cargo test from the workspace root.
• For WASM release builds: cargo build --target wasm32-unknown-unknown --release -p callora-vault (adjust -p if the crate name differs).

Deliverables

• Repair contracts/vault/src/lib.rs and fix test.rs accordingly.
• Run ./scripts/check-wasm-size.sh after release build.

Test and commit

• Run tests and ./scripts/coverage.sh (or cargo tarpaulin per tarpaulin.toml).
• Cover edge cases; include summarized test output and brief security notes in the PR description.

Example commit message

fix(vault): restore compiling vault implementation

Guidelines

• Minimum 95% line coverage for touched crates (workspace policy).
• Clear documentation (Rust /// on public items where applicable; repo markdown as needed).
• Timeframe: 96 hours from assignment unless agreed otherwise.

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[hakymulla]

@hakymulla has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I'd like to apply for this issue. I have open source experience on the stellar ecosystem.

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Congratulations, @hakymulla! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

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[hakymulla]

@greatest0fallt1me the vault crate already compiles cleanly, no clippy error and all tests pass. This issue is not required.