Test that locked and vested always sum to the total

[Glittersup]

Summary

locked and vested are complementary views of the same stream, so their sum should equal total_amount at any point in time. Asserting the invariant directly catches a whole class of arithmetic mistakes that individual boundary tests can miss.

Acceptance criteria

• A test asserts locked plus vested equals the stream total at several points in time.
• The invariant is checked before the cliff, mid-stream, and after the end.
• The suite still passes.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/tricklepay-contracts.git
cd tricklepay-contracts
git remote add upstream https://github.com/TricklePay/tricklepay-contracts.git

Create a branch for this issue:

git checkout -b test/issue-185

Suggested commit message:

test: assert locked and vested sum to the total

Run cargo fmt --check, cargo clippy --all-targets -- -D warnings, and cargo test before opening a pull request and linking this issue.