test: add fee growth direction assertions (closes #606)

[PrazwalR]

Closes #606

Problem

Flipping zeroForOne in the fee growth global storage assignment (Pool.sol:445-449) does not cause any existing test to fail.
This means incorrect fee accounting direction could go undetected.

Solution

Added FeeGrowthDirection.t.sol with tests asserting that after a single-direction swap, feeGrowthGlobal increases only for the input token's tracker, while the other remains zero.

Test Coverage

Test	Scenario
zeroForOne_exactInput	Basic directional check
oneForZero_exactInput	Opposite direction
zeroForOne_exactOutput	Exact-output variant
oneForZero_exactOutput	Exact-output variant
bidirectional_independent	Both directions, trackers remain independent
withTickCrossing_zeroForOne	Swap crosses initialized ticks
withTickCrossing_oneForZero	Swap crosses initialized ticks
fuzz_feeGrowthDirection	1000-run fuzz over direction + amount
native_zeroForOne	ETH pool swap
native_oneForZero	ETH pool swap

Verification

Flipping !zeroForOne → zeroForOne at Pool.sol:445 causes all 10 tests to fail immediately, confirming the tests detect incorrect fee growth direction.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.