dev: make .property.test.ts mean one thing

🧐 Motivation

.property.test.ts currently means two different things.

NativeShieldedTokenPublicSupply.property.test.ts is stateful invariant fuzzing — random op sequences checked against a shadow model. The confidential note token core adopts a narrower split:

• .property.test.ts — stateless properties: generated inputs, single calls, no ledger
• .invariants.test.ts — stateful: generated op sequences plus a shadow model

Two meanings for one suffix is worse than no convention.

📝 Details

• rename NativeShieldedTokenPublicSupply.property.test.ts → .invariants.test.ts (its content is stateful)
• record the convention in GUIDELINES.md § Testing, which already mentions property-based tests but not the file naming

✅ Acceptance criteria

• one meaning per suffix across the repo
• the split is documented, not just implied by filenames

Start by locating NativeShieldedTokenPublicSupply.property.test.ts and GUIDELINES.md § Testing. Review the test's stateful shadow-model behavior, rename it to .invariants.test.ts, document the stateless/stateful naming split, and verify repository references and tests still pass.