Separate minted and boosted shares on vault position

[evgeny-stakewise]

Summary

Pure refactor of the boosted-shares handling in update_redeemable_positions — no behaviour change. The matching logic itself landed in #814; this makes it object-oriented and easier to read.

The core issue was that minted_shares carried two semantically different values: it started as the raw minted amount from the subgraph, then _reduce_boosted_amount destructively subtracted the boosted amount from it, so afterwards the same field meant "shares available for redemption". Everything downstream had to know which phase it was in.

The two values are now separate fields:

• VaultOsTokenPosition.minted_shares stays the raw subgraph value.
• VaultOsTokenPosition.boosted_shares holds the part backing a leverage strategy position.
• VaultOsTokenPosition.redeemable_shares is a derived property: max(minted_shares - boosted_shares, 0).

Follow-on changes:

• _reduce_boosted_amount -> _distribute_boosted_shares: assigns boosted_shares instead of mutating minted_shares, and no longer returns the allocator list it was handed.
• The per-user residual (boosted shares with no same-vault mint to match against) moved from a returned dict into an Allocator.residual_boosted_shares field, so it travels with the object it describes.
• Allocator.total_shares -> total_redeemable_shares, and it and vaults_proportions are now computed from redeemable_shares.
• Variables renamed to match: min_minted_shares -> min_redeemable_shares, address_to_minted_shares -> address_to_redeemable_shares.