test: add fast-check property tests for Stellar scalar arithmetic

[TheDEV111]

Closes #3

Implements all eight properties from issue #3:

• Scalar addition: associativity, commutativity, additive identity
• Round-trip: bytesToScalar(scalarToBytes(a)) == a
• seedToScalar: determinism and distinct-seed collision resistance
• Stealth equation: (m + s_h)G == mG + s_h*G (homomorphism)
• View-tag uniformity: chi-square over 10k sequential inputs
• signWithScalar: signature verification, wrong-message/wrong-key rejection

Default run: 1000 cases. FC_RUNS=100000 via pnpm test:fuzz. Nightly CI job (slow-tests) runs the high-case version at 02:00 UTC.

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[truthixify]

Closing as a duplicate. Issue #3 is being addressed in #15 which was opened first and is currently waiting on a rebase. Sorry about the wasted effort, that's on us for not making assignment-tracking more visible.

Your PR is actually slightly more thorough than #15 in a couple of places. Specifically, you have:

• signature is rejected for wrong message
• signature is rejected for wrong public key
• scalarToBytes always returns 32 bytes (length invariant)
• output is always a valid scalar in [0, L-1] (seedToScalar range invariant)
• deriveStealthPubKey(m*G, s_h) == (m + s_h)*G (alternative formulation of the stealth equation)

Once #15 lands, a small follow-up PR adding these as additional negative-path and invariant tests would be very welcome. Would you be up for that, @TheDEV111? I'll fast-track the review.

Other open Stellar Wave issues you'd be well-suited for, given the property-test rigor here:

• sdk Worker pool for parallel multichain scanning #25 (worker pool for parallel multichain scanning)
• sdk Long-running scan memory profiling + leak detection #26 (long-running scan memory profiling + leak detection)
• contracts Stellar SDK benchmark suite + baseline report #5 (additional Stellar Soroban property tests)