[38] Add Soroban token-allowance withdrawal pattern for QuestEngine employer-funding

[merlik787-droi]

Problem Statement

quest-engine::create_build_quest requires the employer to call with sufficient balance and approve the transfer at the same time. This forces employers to send a transaction with their balance at the moment of quest creation. If the create_build_quest call is assembled with the wrong amount or wrong token, the whole call reverts.

A pattern where the employer could pre-authorize the QuestEngine to pull funds on demand (Soroban token allowance) would be more flex-protocol-friendly.

Why It Matters

• Reduces friction for employer integration.
• Allows multi-quest setup with a single allowance.

Expected Outcome

• New function set_escrow_allowance(env, employer, max_amount) (an internal flow — actually, this is just the SAC token allowance, so this concerns caller UX, not contract invocations).
• Add a helper function create_build_quest_with_allowance that uses token::Client::transfer_from (requires SAC support for allowances) if and when the network supports it.

Acceptance Criteria

• Document uses the helper pattern in employer guides.
• Tests for the allowance flow.

Implementation Notes

• Soroban's SAC token may or may not support allowances depending on protocol version.
• Reference https://developers.stellar.org/docs/tokens/stellar-asset-contract

Files / Modules Affected

• contracts/quest-engine/src/lib.rs
• contracts/quest-engine/README.md

Dependencies

Requires SAC token allowance support.

Difficulty

Medium.

Estimated Effort

1–2 days.

Suggested Labels

feature, ux, P3, quest-engine

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[grantfox-oss]

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