Swap leaves to ensure quoted leave amount does not change above limit

[danielgranhao]

Replaces #730. Closes #697 (resolving the underlying issue using a different strategy from what was suggested in the issue)

When an on-chain withdrawal fee quote is fetched, the SSP bases fees on the current leaf count. If leaves change between quoting and sending (e.g., due to incoming payments splitting leaves), the leaf count may exceed the SSP's tolerance, causing the withdrawal to fail.

This PR:

• Records the quoted leaf count when fetching a fee quote
• Computes the SSP's max allowed leaf count using ceil(q * (1.2 + 1.0 / q^0.6))
• Passes a max_amount_leaf_count constraint through leaf selection, forcing a consolidation swap if needed
• Rejects the withdrawal if the swap still can't bring the count within tolerance
• Adds an integration test covering this scenario

[roeierez]

Nice and simple!

[danielgranhao]

Added a commit for improving the behavior when FeesIncluded is used. There was a relatively high risk of issues because, in this case, we want to send an amount less than the initially quoted amount. For certain values and depending on the fee, this makes it impossible to respect the max. Example: try to send 16384 sats (power of 2) -> we use a single leaf for the quote -> we try to send (16384 - selected fee), which is practically guaranteed to use more than 2 leaves (max allowed by SSP).

To fix this, I've changed the amount we use to get the quote. When fees are included, we use a worst case value: prev_power_of_2(amount) - 1 (e.g. for 16384, we quote for 16383). This value has the maximum number of set bits (= popcount) that any value below amount can have, so any amount - fee is guaranteed to need at most that many leaves. This way, the SSP's leaf count tolerance is always sufficient regardless of the fee. This works because SSP quotes are not tied to the amount that is sent.

[roeierez]

LGTM

[danielgranhao]

We've discussed with Lightspark and it seems they may soon remove the leaf count change quote validation, making this unnecessary. Closing.