test(events): grant pillar — claim_milestone dynamic-release math

[0xdevcollins]

Part of #25 (contract test-coverage push).

events

How to test

• Reuse the crate's tests/common.rs harness; add tests/grant_pillar.rs + a one-line mod grant_pillar; in tests/mod.rs.
• Template: contracts/events/src/tests/crowdfunding.rs.
• Run cargo test -p boundless-events, then cargo test --all before the PR.

Acceptance criteria

• New tests/grant_pillar.rs covering every function in scope.
• Happy path + each Error variant + edge cases + auth-rejection (+ idempotency where relevant).
• cargo test --all passes; PR links this issue.

[dev8967]

Implemented in PR #42: #42

[legend4tech]

Hello, I’m Legend4Tech, a full-stack developer and top open-source contributor, previously a fellow at OnlyDust. I’m interested in contributing to this issue and would be happy to help implement a solution, fix bugs, or improve features as needed. I’m ready to follow the project’s workflow and coding standards to ensure smooth collaboration.