feat: permissionless liquidation trigger for defaulted invoices

[Dannyswiss1]

Title: feat: permissionless liquidation trigger for defaulted invoices

Suggested branch: feat/1037-permissionless-liquidation-trigger

Body:

Summary

First slice of the end-to-end liquidation pipeline proposed in #1037: a permissionless keeper entrypoint for the collateral-seizure step of the state machine.

• Add PoolContract::liquidate_invoice(caller, invoice_id) — any address can trigger it once an invoice is objectively Defaulted on the invoice contract. Unlike the existing seize_collateral, there's no admin/governance-proposal gate on the trigger itself.
• Refactor the seizure logic into a shared seize_collateral_core helper so both the governance path (execute_seize_collateral) and the new permissionless path share one idempotency guard (CollateralDeposit.settled) and one non_reentrant! lock — a defaulted invoice cannot be double-liquidated through either path.
• Emit distinct liq_started / liq_seized events (separate from the existing col_seiz_default) so the indexer can later distinguish keeper-triggered liquidations for pipeline-stage tracking.
• packages/sdk: add PoolClient.liquidateInvoice() (the keeper-trigger call) and PoolClient.getCollateralDeposit() (a liquidation-status query), plus the CollateralDeposit type.

This covers only the "mark defaulted → seize collateral" step. Still open, per #1037's full acceptance criteria:

• Insurance draw-down wiring (insurance.file_claim as a keeper-chained follow-up call — pool can't call it atomically due to Soroban's pool→insurance→pool re-entrancy restriction, noted in the code)
• Auction clearing for seized non-cash collateral
• Senior/junior tranche loss-ordering on liquidation
• Frontend "Defaulted invoices" keeper/investor views
• Indexer event handling + liquidation-history endpoint
• The two full pipeline integration tests (simple + tranched) named in the acceptance criteria

Test plan

• cargo test -p pool --lib liquidate_invoice — 3/3 new tests pass (permissionless happy path, rejection when not yet defaulted, double-liquidation rejection via both permissionless and governance paths)
• cargo test -p pool --lib seize_collateral — all 6 existing tests still pass (no regression from the refactor)
• tsc --noEmit clean in packages/sdk
• packages/sdk Jest suite passes (4/4)

Closes #1037