Protect subscription charges against reentrancy

[TUPM96]

Summary

• add a shared ReentrancyLock(charge_subscription) storage key and guard the full charge flow
• reorder charge_subscription to persist subscription/revenue effects before token and invoice cross-contract calls
• add malicious-token callback regression coverage, a property test for reentrant charge attempts, and update the threat model/spec notes

Fixes #423

Validation

• cargo test --workspace
• cargo build --workspace --target wasm32v1-none --release

Both pass. The build still emits existing baseline warnings from the gas profiling/storage modules.

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