fix(auth-utils): move destroyed check inside mutex for atomic validation

CRITICAL FIX: Addresses Copilot AI comment on PR #79 about race condition
between destroyed flag check and mutex acquisition.

Problem:
The destroyed flag check happened OUTSIDE the mutex, creating a race window
between check and mutex acquisition. destroy() could complete after check
passes but before mutex is acquired, leading to transaction using destroyed
resources.

Timeline before fix:
T0: transaction() line 307 - check destroyed (false) ✅
T1: destroy() line 350 - destroyed = true
T2: destroy() line 379 - preKeyManager.destroy()
T3: transaction() line 324 - mutex.runExclusive() acquires mutex
T4: work() executes → uses destroyed resources → CRASH

Changes:
- Removed destroyed check from line 306-309 (outside mutex)
- Added destroyed check inside mutex.runExclusive() at line 320-324
- Check now happens atomically within mutex protection
- If mutex acquired, resources guaranteed to exist

Timeline after fix:
T0: transaction() line 315 - getTxMutex(key)
T1: destroy() line 350 - destroyed = true
T2: destroy() line 358 - checks mutex (locked by transaction)
T3: destroy() line 370 - early return (resources NOT destroyed)
T4: transaction() line 319 - mutex.runExclusive() executes
T5: transaction() line 322 - check destroyed → true → throws error ✅

OR (if check happens first):
T0: transaction() line 319 - mutex.runExclusive() acquires mutex
T1: transaction() line 322 - check destroyed (false) ✅
T2: destroy() called → line 358 checks mutex (LOCKED)
T3: destroy() early returns (resources safe)
T4: transaction() completes successfully

Validation:
- ✅ Check is now atomic (inside mutex critical section)
- ✅ No window between check and resource usage
- ✅ destroy() respects locked mutex (existing protection)
- ✅ Either transaction completes OR gets rejected, never crashes

https://claude.ai/code/session_VMxqX

src/Utils/auth-utils.ts

@@ -303,11 +303,6 @@ export const addTransactionCapability = (
 		isInTransaction,
 
 		transaction: async (work, key) => {
-			// CRITICAL: Prevent transactions after destroy to avoid use-after-free
-			if (destroyed) {
-				throw new Error('Transaction capability destroyed - socket closed')
-			}
-
 			const existing = txStorage.getStore()
 
 			// Nested transaction - reuse existing context
@@ -322,6 +317,12 @@ export const addTransactionCapability = (
 
 			try {
 				return await mutex.runExclusive(async () => {
+					// CRITICAL: Check destroyed flag INSIDE mutex to prevent race condition
+					// This ensures atomic check-and-execute: if we acquire mutex, resources exist
+					if (destroyed) {
+						throw new Error('Transaction capability destroyed - socket closed')
+					}
+
 					const ctx: TransactionContext = {
 						cache: {},
 						mutations: {},