Enforce z - zeta >= z_min (except for LPs)

[jalextowle]

Fixes: #583, #677.

This PR enforces $z - \zeta \geq z_{min}$ for trading operations but continues to allow LP operations like removeLiquidity and the system level _distributeExcessIdle to bring the effective share reserves below the minimum share reserves.

As discussed theoretically in #583 and practically in #677, it is quite dangerous for the system's liveness to allow traders to manipulate the effective share reserves to be very small. A sane cutoff for this manipulation is the minimumShareReserves since this was selected to be a sane minimum for the share reserves which is subject to similar constraints. Enforcing this check is straightforward in closeLong and openShort. Since the LP operations can violate the invariant, we only enforce $z - \zeta \geq z_{min}$ in closeLong when the effective share reserves are decreasing to allow checkpoints to be minted regardless of the current effective share reserves.

The idea of enforcing $z - \zeta \geq z_{min}$ seems reasonable on its face, but it prevents LPs from being able to remove all of the shares from system even if there are no outstanding positions. Attempting to enforce this check would complicate the logic in LPMath.calculateDistributeExcessIdle and would also create situations in which LPs wouldn't be able to remove non-trivial amounts of liquidity. For example, assume that $\zeta > 0$ and $\tfrac{z}{z - \zeta} = 50$. If we enforced $z - \zeta \geq z_{min}$, then $z - \zeta = z_{min}$ after removing the liquidity but $z = 50 \cdot z_{min}$. This is untenable as the ratio increases. With all of this in mind, this PR doesn't enforce the constraint for LPs and instead adds a test demonstrating that liveness is not compromised by LPs removing all of their liquidity even in extreme trading conditions.

In order for the calculateMaxShort machinery to respect the $z - \zeta \geq z_{min}$ constraint, I needed to update the rounding behavior. I also took the opportunity to simplify $\max(z_{min}, z_{min} + \zeta)$ to $z_{min} + \max(\zeta, 0)$.

[github-actions]

Hyperdrive Gas Benchmark

Benchmark suite	Current: 0371ec0	Previous: 2a8c81f	Deviation	Status
addLiquidity: min	1633 gas	1600 gas	2.0625%	🚨
addLiquidity: avg	66533 gas	64292 gas	3.4857%	🚨
addLiquidity: max	291577 gas	275113 gas	5.9845%	🚨
checkpoint: min	1150 gas	1150 gas	0%	🟰
checkpoint: avg	47770 gas	47670 gas	0.2098%	🚨
checkpoint: max	190769 gas	190992 gas	-0.1168%	✅
closeLong: min	1558 gas	1558 gas	0%	🟰
closeLong: avg	28050 gas	28087 gas	-0.1317%	✅
closeLong: max	150962 gas	148328 gas	1.7758%	🚨
closeShort: min	1549 gas	1549 gas	0%	🟰
closeShort: avg	29758 gas	29855 gas	-0.3249%	✅
closeShort: max	125358 gas	147421 gas	-14.9660%	✅
initialize: min	1582 gas	1538 gas	2.8609%	🚨
initialize: avg	213188 gas	213159 gas	0.0136%	🚨
initialize: max	255056 gas	254817 gas	0.0938%	🚨
openLong: min	1487 gas	1487 gas	0%	🟰
openLong: avg	50377 gas	50366 gas	0.0218%	🚨
openLong: max	184813 gas	185024 gas	-0.1140%	✅
openShort: min	1608 gas	1608 gas	0%	🟰
openShort: avg	49989 gas	49663 gas	0.6564%	🚨
openShort: max	180216 gas	179974 gas	0.1345%	🚨
redeemWithdrawalShares: min	1575 gas	1575 gas	0%	🟰
redeemWithdrawalShares: avg	19360 gas	20332 gas	-4.7806%	✅
redeemWithdrawalShares: max	57733 gas	106163 gas	-45.6185%	✅
removeLiquidity: min	1639 gas	1639 gas	0%	🟰
removeLiquidity: avg	149448 gas	148128 gas	0.8911%	🚨
removeLiquidity: max	323845 gas	323757 gas	0.0272%	🚨

This comment was automatically generated by workflow using github-action-benchmark.

[coveralls]

coverage: 95.216% (+0.007%) from 95.209%
when pulling 0371ec0 on jalextowle/chore/effective-share-reserves-greater-than-minimum-share-reserves
into 6b3cb74 on main.

[jrhea]

lgtm