Turn the auction contract into a real competitive discount-funding mechanism

[sanmipaul]

Problem

contracts/auction exists in the workspace but has no live caller. Its only integration point into pool was fund_invoice_with_discount, which was removed as confirmed dead code during a recent CI-stabilization pass — a grep across the workspace and frontend/SDK turned up zero remaining references to the auction contract anywhere. It currently sits in the repo as an unreachable skeleton: no bidding lifecycle, no escrow, no settlement path, no tests beyond the bare minimum.

Proposed scope

Build out auction into a real competitive discount-funding mechanism for invoices, integrated with pool via cross-contract calls (mirroring how the new secondary_market satellite contract calls back into pool's trusted market_settle_listing entrypoint for settlement — see contracts/pool/src/lib.rs and contracts/secondary_market/src/lib.rs).

Work required:

• Contract: design and implement the actual auction mechanics — opening an auction for an invoice at a target discount/rate, investors placing competitive bids (with bid escrow so funds are locked, not just promised), a close/settlement path that picks the winning bid(s) and funds the invoice through a trusted cross-contract call into pool (new auction-side settlement entrypoint on pool, following the market_settle_listing trust pattern: satellite calls in as itself, pool compares against a stored trusted address), bid withdrawal/refund for non-winning bids, and auction expiry handling.
• Storage/state: bids, auction lifecycle status, per-investor escrowed amounts — sized and indexed so listing/settlement stays bounded (cap on bids per auction, matching the MAX_LISTINGS_PER_INVOICE-style bound used elsewhere in this codebase).
• Tests: full lifecycle coverage (auction open → bids placed → close → settlement, competing bids at different discount rates, expiry with no bids, expiry with unfilled capacity, bid withdrawal, double-bid/self-bid rejection, unauthorized settlement attempts) plus fuzz/property tests for the bid-ranking and settlement math, following the pattern in contracts/pool/tests/fuzz_tests.rs.
• SDK: add an AuctionClient to packages/sdk/src/clients/ (mirroring PoolClient/the new secondary-market client), covering open/bid/withdraw/close/query methods, and reflect it in sdk/'s built output.
• Frontend: UI for browsing open auctions, placing/withdrawing a bid, and viewing an invoice's auction history, wired through frontend/lib/contracts.ts with a NEXT_PUBLIC_AUCTION_CONTRACT_ID env var.
• Indexer: parse and store the auction's emitted events (open/bid/close/settle/expire) in indexer/src/parser.ts, following the existing event-namespace pattern used for pool/secondary-market events.

Acceptance criteria

• An SME can have an invoice funded via a competitive auction end-to-end: open auction → multiple investors bid → auction closes → winning bid(s) settle through pool → invoice is funded, all exercised in integration tests.
• Non-winning bids are refunded (escrowed funds released) on settlement or expiry.
• cargo build --target wasm32-unknown-unknown --release produces an auction.wasm under Soroban's 200KB limit, with the CI wasm-size check green.
• No regression in cargo clippy -- -D warnings or cargo fmt --check.
• Frontend auction flows work end-to-end against a deployed auction contract on testnet, and the indexer correctly ingests all auction events.