Make the hook count swaps.

_beforeSwap increments unconditionally, before the pool has computed anything. v4 does not reject a swap against a pool with no in-range liquidity: it walks the price to the limit, moves no value, returns a zero BalanceDelta and does not revert. That call is counted. The same holds for a swap so small the LP fee rounds the fill to zero.

Because beforeSwap runs ahead of Pool.swap, the hook cannot distinguish these from a real trade without reading the swap result, which needs afterSwap - out of scope for this task, which asked for one counter and one getter. Reported rather than asserted silently because the tests that cover this case pin the current behaviour, and 'swapCount' reads like a count of trades.

Two consequences for anything that later consumes this number: it is an upper bound on trades, not a count of them; and since anyone may swap on any pool, and anyone may open their own pool on a public hook, it is cheap for an outsider to move.

Neither is a departure from the stated task - the task says 'increment once per swap' and a swap call is what is being counted - but a caller keying a decision off swapCount (a launch milestone, a fee schedule, an unlock) would be reading it wrong.