test(host-contracts): document HCULimit invariants in plain english

Author: Eikix

host-contracts/test/hcuLimit/HCULimit.invariants.t.sol

@@ -176,24 +176,29 @@ contract HCULimitInvariantTest is StdInvariant, Test {
     }
 
     function invariant_blockMeterMatchesCurrentBlock() public view {
+        // Invariant: getBlockMeter must always report the current block number.
         (uint64 blockNumber,) = hcuLimit.getBlockMeter();
         assertEq(blockNumber, uint64(block.number));
     }
 
     function invariant_blockMeterUsesCastGranularity() public view {
+        // Invariant: accounted block HCU is made of Uint8 cast ops (32 HCU each), so it stays a multiple of 32.
         (, uint192 usedHCU) = hcuLimit.getBlockMeter();
         assertEq(uint256(usedHCU) % 32, 0);
     }
 
     function invariant_whitelistedCallsDoNotConsumePublicMeter() public view {
+        // Invariant: calls from whitelisted callers must never increase the public block meter.
         assertFalse(handler.whitelistViolation());
     }
 
     function invariant_nonWhitelistedAccountingIsConsistent() public view {
+        // Invariant: for non-whitelisted callers, meter growth equals successful op count times cast-op cost.
         assertFalse(handler.nonWhitelistedAccountingViolation());
     }
 
     function invariant_meterResetsAfterMinedBlocks() public view {
+        // Invariant: once the block number advances, observed usedHCU for the new block is zero until a new op succeeds.
         assertFalse(handler.resetViolation());
     }