chore: commit Cargo.lock and enforce locked CI builds

[Jayydy]

Description

Summary

Commits contracts/Cargo.lock to version control and updates CI to use
--locked builds, ensuring reproducible dependency resolution for the
token-factory Soroban contract.

Changes

• contracts/Cargo.lock — generated and added to the repository
• .github/workflows/wasm-build.yml — added --locked flag to the
  cargo build step

Why

The token-factory contract is a deployable binary artifact (cdylib).
Per Rust best practices, application/binary crates should commit their
Cargo.lock so every build — local or CI — resolves the exact same
dependency versions. Without --locked, CI could silently pick up
newer patch versions that differ from what was tested locally.

Verification

• git check-ignore -v contracts/Cargo.lock exits non-zero (file is not ignored)
• contracts/Cargo.lock is present and tracked
• CI cargo build --locked will fail fast if the lockfile is ever
  missing or out of sync with Cargo.toml
  .

Related Issue

Link the issue using "Closes #XX" or "Fixes #XX".

Closes #84

Type of Change

Please tick the relevant options:

• Bug fix (non-breaking change which fixes an issue)
• [ x] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Testing Done

Please describe the tests you ran (unit tests, manual verification, etc.) to verify your changes.

Checklist

• [ x] My code follows the style guidelines of this project
• [ x] I have performed a self-review of my own code
• [x ] I have made corresponding changes to the documentation
• [x ] My changes generate no new warnings
• [ x] I have added tests that prove my fix is effective or my feature works
• [ x] New and existing unit tests pass locally with my changes

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