Fee curve update

[aalavandhan]

Updated enrichment/debasement fee curve.

Motivation:

The current implementation caps the enrichment rate when dr is high. The side-effect of this is that the vault's effective funding rate (rate vault holders pay to get magnified exposure) reduces.

When dr = 2, for example the perpTVL:vaultTVL ratio is 1:6 and at dr = 3 its 1:9. Say there's a perp tvl of 1m$. Perp's enrichment would be ~200k at dr = 2 and at dr = 3. However, the vault's effective rate would be 200k/6m ~3.3% when dr =2 and 200k/9m ~2.2% when dr = 3 and so on. So it actually continues to go down. (As shown in the following figure)

Alternatively as dr continues to increase we want the vault's effective funding rate to increase or at least stay constant. This would mean that perp's enrichment rate is unbounded on the upside.
(As shown below)

[openzeppelin-code]

Fee curve update

Generated at commit: 36e93acb815b72025efeb17ad592c2d61fb37ef7

🚨 Report Summary

	Severity Level	Results
Contracts	Critical High Medium Low Note Total	1 0 0 3 23 27
Dependencies	Critical High Medium Low Note Total	0 0 0 0 0 0

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For more details view the full report in OpenZeppelin Code Inspector

[brandoniles]

Let's call this something like negPerpRate (most accurate?) so it's more clear what this intermediate value is.

Then, maybe take the max of -1* negPerpRate and the debasement limit? Seems more in the spirit of the computation.

[brandoniles]

LGTM