Better socialising of losses

[naddison36]

Summary

This PR improves the Model A loss-sharing implementation by simplifying withdrawal queue accounting and making claimability share-denominated.

The main change is replacing the asset-side cumulative counters withdrawsQueued and withdrawsClaimed with a single reservedWithdrawLiquidity value. This value represents the maximum amount of liquidity assets that must remain reserved for outstanding withdrawal requests.

Changes

• Replaced withdrawsQueued / withdrawsClaimed with reservedWithdrawLiquidity.
• requestRedeem increases reservedWithdrawLiquidity by the request-time asset amount.
• claimRedeem decreases reservedWithdrawLiquidity by request.assets, even if the actual loss-adjusted payout is lower.
• Kept FIFO queue accounting share-denominated via withdrawsQueuedShares and withdrawsClaimedShares.
• Changed claimable() to return the cumulative claimable share frontier instead of liquidity assets.
• Updated claimRedeem to check request.queued <= claimable().
• Simplified _availableAssets() so it only returns available assets.
• Moved withdrawal reservation handling into the specific functions that need it, especially _allocate.

Claimability Semantics

claimable() now returns a cumulative share amount:

withdrawsClaimedShares + convertToShares(claimableLiquidity)

This means a UI can determine whether a request is claimable with:

request.queued <= claimable()

assuming the request is not already claimed and its claim delay has elapsed.

Accounting Model

This keeps the two accounting domains separate:

• reservedWithdrawLiquidity is asset-denominated and protects liquidity for outstanding withdrawal requests.
• withdrawsQueuedShares and withdrawsClaimedShares are share-denominated and define FIFO queue position.

This makes the pro-rata loss-sharing model easier to reason about while preserving the claim-time payout cap:

min(request.assets, convertToAssets(request.shares))

Notes

This PR changes the public meaning of claimable() from asset liquidity to a share-denominated queue frontier.

[naddison36]

Closing in preference for #223