fix(SuperchainConfig): extend() must check paused() to prevent re-activating expired pauses

[Sertug17]

Fixes #391

Problem

extend() checked pauseTimestamps[_identifier] == 0 to detect "not paused", but this only catches the never-paused case. An expired pause has pauseTimestamps != 0 while paused() returns false, so it silently passed the check and got its timestamp reset — effectively re-pausing the system without the required unpause() → pause() cycle.

This bypasses the Stage 1 Decentralization requirement explicitly documented in pause():

"this check intentionally prevents re-pausing even after a pause has expired... This is a Stage 1 Decentralization requirement"

Fix

- if (pauseTimestamps[_identifier] == 0) {
+ if (!paused(_identifier)) {

paused() returns false for both never-paused (timestamp == 0) and expired (timestamp != 0 but past PAUSE_EXPIRY), making it the correct guard.

Missing test (to be added)

function test_extend_expiredPause_reverts() external {
    _pauseAsGuardian(address(this));
    vm.warp(block.timestamp + PAUSE_EXPIRY + 1);
    assertFalse(superchainConfig.paused(address(this)));

    vm.prank(superchainConfig.guardian());
    vm.expectRevert(
        abi.encodeWithSelector(
            ISuperchainConfig.SuperchainConfig_NotAlreadyPaused.selector,
            address(this)
        )
    );
    superchainConfig.extend(address(this));
}

[cb-heimdall]

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