Add stablecoin supply invariant test

[MyTH-zyxeon]

Summary

• add a Foundry invariant handler for Stablecoin mint/burn sequences
• assert totalSupply always equals cumulative mints minus burns
• wire Foundry to use forge-test and vendor the forge-std source needed by existing tests

Fixes #25

Testing

• forge test --mt invariant_ -vvv
• forge test

[abhicris]

🤖 Audit skipped — diff is 956KB (limit 64KB). Labelling for human review.

[MyTH-zyxeon]

Updated to address the human-review label from the large diff audit.

Changes in the refreshed branch:

• removed the vendored forge-std tree from the PR
• replaced it with a tiny local test helper for the existing Foundry tests
• kept the new invariant test focused on totalSupply == net mint/burn accounting

Verification rerun locally:

• forge test --mt invariant_ -q
• forge test -q

The branch diff is now 3 files / 180 insertions instead of ~956KB.