test(cip-29): reduce inflation

[rootulp]

cc: @tac0turtle

[rootulp]

Discussed w/ @tac0turtle offline and this PR currently assumes that the constants for initial inflation rate and disinflation rate decreased by 33% but the CIP is actually suggesting retaining the constants as is and then decreasing them by 33% when calculating inflation. It means we can't use this formula any more:

InitialInflationRate * ((1 - DisinflationRate) ^ YearsSinceGenesis)

or rather the formula needs a conditional where it selectively switches based on the app version.

[rootulp]

@tac0turtle when I use the reference implementation in CIP-29, the inflation numbers diverge from what is presented in the table. I'm observing

		testCases := []testCase{
			{0, 0.0536}, // note this value won't be used in production because celestia-app was on app version 3 in year 0.
			{1, 0.04824},
			{2, 0.040507128},
			{3, 0.0364564152},
		}

when the table says https://github.com/celestiaorg/CIPs/pull/249/files#diff-5bc579390f911ecfc45fde43055ce471aea57812b146e1669a507682deb3ef15R57-R59

[tac0turtle]

here the math is slightly off same with the code in the CIP.

Here we are not doing a reduction of the previous year inflation we are taking the original inflation rate for that year reducing it by 33% and then reducing it by 6.7%

in order to calculate the correct inflation we would need to do something like so

func calculateInflationRatePostCip29(ctx sdk.Context, genesis time.Time) sdk.Dec {
	if ctx.ConsensusParams().Version.AppVersion <= 3 {
		panic("calculateInflationRatePostCip29 should not be called with AppVersion <= 3")
	}

	years := yearsSinceGenesis(genesis, ctx.BlockTime())
	initialInflationRate := InitialInflationRateAsDec().Mul(sdk.OneDec().Sub(DisinflationRateAsDec()).Power(uint64(1)))

	updatedInflationRate := sdk.Dec{}
	// For AppVersion > 3, adjust the inflation rate:
	if ctx.ConsensusParams().Version.AppVersion > 3 {
		// First, reduce the current inflation rate by 33%
		updatedInflationRate = initialInflationRate.Mul(sdk.NewDecWithPrec(67, 2)) // 0.67 \= 67%

		// Then, if we are in year two or later, apply a one-time disinflation of 6.7%
		if years >= 2 {
			for i := 1; i < int(years); i++ {
				updatedInflationRate = updatedInflationRate.Mul(sdk.OneDec().Sub(sdk.NewDecWithPrec(67, 3))) // 1 \- 0.067 \= 0.933
			}
		}
	}

	// Ensure the inflation rate does not fall below the target inflation rate.
	if updatedInflationRate.LT(TargetInflationRateAsDec()) {
		return TargetInflationRateAsDec()
	}

	return updatedInflationRate
}

the code isnt the nicest, would be happy to simplify and change the numbers as they are not that far off.