test: add Stellar scalar property coverage

[kaycke1337]

Closes #3

Summary

• adds fast-check property coverage for Stellar scalar arithmetic, seed derivation, stealth public-key equations, view-tag distribution, and signWithScalar verification
• adds test:properties and high-case test:fuzz scripts
• documents property testing in the README
• adds a nightly slow-tests CI job for the 100,000-case fuzz run

Validation

• npx --yes pnpm@10.28.2 test:properties
• npx --yes pnpm@10.28.2 format:check
• npx --yes pnpm@10.28.2 build
• npx --yes pnpm@10.28.2 test

Commit used --no-verify because the local hook calls a global pnpm binary that is not installed in this environment; the same commands were run through npx pnpm.

[truthixify]

Heads up — we've just set up a develop branch as the integration target for all Drips × Stellar Wave contributions, and I've retargeted this PR's base from main to develop on your behalf. The diff is unchanged; nothing to do on your end.

Going forward, please branch from and target develop. We'll periodically merge develop → main as we cut releases. Thanks for contributing!

[truthixify]

All 8 properties from the issue spec are implemented (with the chi-square view-tag test as a nice touch), WRAITH_FUZZ_RUNS env knob, nightly CI at 100k cases. Merging. Thanks @kaycke1337.

[truthixify]

Hmm, my earlier merge call failed: conflicts surfaced after #14 landed (likely package.json scripts overlap with my comment timing). Could you rebase onto develop? Once clean I'll merge immediately, content is good.

git fetch origin
git rebase origin/develop
git push --force-with-lease