chore: partial read for asset balances

[RoeeGross]

Note

Introduce syscall-based partial read access for Option<AssetBalance> and switch get_synthetic_balance to use it for efficient balance retrieval.

• Core types (core/types/position.cairo):
  • Add OptionAssetBalanceReadAccessTrait with syscall-based partial reads (read, is_some/is_none, get_balance, get_funding_index, get_version).
  • Define OptionAssetBalanceOffset enum with offset mapping for storage access.
  • Import low-level storage utilities to support direct field reads.
• Positions component (core/components/positions/positions.cairo):
  • Refactor get_synthetic_balance to use OptionAssetBalanceReadAccessTrait::get_balance on the iterable map entry pointer instead of reading the full Option<AssetBalance>.
  • Update iterable map imports to include IterableMapTrait.

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[MohammadNassar1]

@MohammadNassar1 reviewed 1 of 2 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @remollemo)