test: property-based coverage for amount validator

[Ope4196]

PR: Property-based tests for amountValidator (Stellar/USDC precision)

Summary

Adds 11 fast-check property-based tests to src/validators/amountValidator.test.ts, expanding coverage beyond the existing unit tests to fuzz fractional precision, leading zeros, negative sentinels, and denormalized number strings so silent regressions are caught automatically.

Changes

Modified files

• src/validators/amountValidator.test.ts — added 11 fc.property tests (PBT-1 through PBT-11), all with pinned seeds for determinism

Properties covered

#	Property	Requirement
PBT-1	All valid canonical amounts accepted	Baseline validity
PBT-2	normalizedAmount equals input for valid amounts	Round-trip identity
PBT-3	toSmallestUnit stroop → string → stroop round-trip	Bigint correctness
PBT-4	Integer-equivalent amounts (N.0000000) round-trip	Issue requirement
PBT-5	>7 fractional digits always rejected	Issue requirement
PBT-6	Negative sentinel strings always rejected	Issue requirement
PBT-7	Leading zeros in fractional part handled correctly	Precision boundary
PBT-8	Scientific notation strings always rejected	Stellar/USDC format
PBT-9	Whitespace-padded strings always rejected	Format strictness
PBT-10	NaN/Infinity string variants always rejected	Denormalized inputs
PBT-11	toSmallestUnit always returns positive bigint	Stroop correctness

Design notes

• All fc.assert calls use { seed: 1234567 } for reproducibility — no flaky seeds
• validAmountArb generates amounts from stroop integers via stroopsToCanonical, guaranteeing exact IEEE 754 representation with no precision loss
• numRuns kept to 200–500 per property; total runtime well under 5 s
• fast-check is already a devDependency — no new dependencies added

Validation

npm test -- --testPathPattern=amountValidator

closes #420