fix: serialize balance-check-and-transfer to close TOCTOU race (CWE-367)

[Matthew-Selvam]

Summary

Fixes #177. transfer_credits (and the identical pattern in fund_child) check getCreditsBalance() and call transferCredits() as two separate awaits with no atomicity between them:

T1: balance = $100, amount = $40 → passes guard (40 < 50) ✓
T2: balance = $100, amount = $40 → passes guard (40 < 50) ✓
T1: transfers $40 → balance now $60
T2: transfers $40 → balance now $20 (violated 50% limit!)

Fix

Adds a per-agent-process credit transfer mutex (withCreditTransferLock, a simple serialized promise chain) around the balance-check-and-transfer critical section in both transfer_credits and fund_child, so a second concurrent call re-checks the guard against the post-transfer balance instead of a stale one. A failed transfer inside the lock doesn't wedge it for later callers.

As the issue notes, a server-side atomic transfer would be the ideal long-term fix — that's out of scope here since it lives in Conway's backend, not this client. This closes the client-side race in the meantime.

Testing

• tsc --noEmit passes.
• Added a regression test: two concurrent $40 transfers against a $100 balance. Exactly one now succeeds and the other is correctly blocked once the balance drops to $60 (where $40 exceeds the new $30 half-balance limit) — previously both would have gone through.
• Full tools-security.test.ts (72/72) and authority-rules.test.ts (36/36, covers fund_child) pass.

[Matthew-Selvam]

This is ready for maintainer review whenever you get a chance — happy to make any changes requested.