Add yield harvesting

[jribbink]

Closes #24

Adds yield harvesting as a leg of rebalance(): when the yield factor (yield value / debt) rises above the immutable yieldFactorMax band edge, sells the surplus yield down to the debt backing and supplies it as collateral (add-only), deploying the gains back into the position. Zero-amount guards skip dust so it can't revert the rebalance. (The band's lower edge — yieldFactorMin + a delever to restore backing on a depeg — is a stacked follow-up.)

Also moves setMaxSlippageBps to onlyOwner, matching the other config setters.

[jordanschalm]

Suggested change

	/// health-factor band bounds. The lower edge (`yieldFactorMin` + a delever) is a
	/// stacked follow-up.
	/// health-factor band bounds.

I think this is talking about a followup PR (?). If so probably doesn't need to be in the comments

[jordanschalm]

@Gornutz Do you think we can expect to have a direct yield <-> collateral swap pool? Or is it better to do two consecutive swaps using the already-configured pools, as the implementation does here.

[Gornutz]

Most of the time I wouldn't expect there to be a yield token <-> collateral pool to exist. Thinking about it from how much capital we would need to keep on those pools make it quite inefficient at the moment. At the moment I do think that having 2 leg swap is probably the best path forward. Then a slightly better/ optimal path is using redemption for yield token -> deposit asset followed by a swap from deposit asset to collateral asset, just so that you are slightly decreasing slippage/swap fees.