feat: get_claimable_funding_amount view for active position holders

[abayomicornelius]

Summary

Position holders cannot currently see how much funding they owe or are owed without simulating a position update. Add a read-only view that returns the pending funding amount for a given position.

Proposed API

// reader contract
pub fn get_claimable_funding_amount(
    env: Env,
    data_store: Address,
    oracle: Address,
    position_key: Bytes,
) -> FundingAmountResult

pub struct FundingAmountResult {
    pub long_token_amount: i128,  // positive = owed to holder, negative = holder owes
    pub short_token_amount: i128,
    pub at_ledger: u64,
}

Acceptance Criteria

• View computes pending funding from current funding_amount_per_size minus position's last-settled index
• Returns signed values: positive = receive, negative = pay
• Does not modify any state
• Unit test: verify returned amount matches what is deducted during actual position decrease
• Frontend integration note: call before showing 'close position' confirmation

Affected Contracts

reader, position_utils

[victor62-art]

@victor62-art has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

pls assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @victor62-art to this issue.

[0xElyte]

@0xElyte has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I'd love to resolve this fast!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @0xElyte to this issue.

[Adamsonoladipupo]

@Adamsonoladipupo has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this issue because it align with my skill set.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Adamsonoladipupo to this issue.