Add `Makefile` target for fuzz tests

[Levi-Ojukwu]

Description:

The Makefile has no target to run the fuzz test suite (iln_fuzz crate). Fuzz tests are important for finding edge cases and should be easily runnable.

Why it matters: Fuzz tests are expensive to run and easy to forget. A Makefile target ensures they're included in CI.

Acceptance Criteria:

• Add test-fuzz target that runs cargo test -p iln_fuzz
• Document it in the help target

Relevant Files: Makefile

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[devJaja]

I'm an experienced software developer
I can solve this issues, ETA 12 hours

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #430)
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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@devJaja's PR #436 was approved and merged by @Levi-Ojukwu.

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👏 Great work, @devJaja! Keep contributing to Invoice-Liquidity-Network.