feat(stellar): tx seam, escrow FSM, payout taxonomy, smoke test

[rougepandaq]

Closes #259, closes #256, closes #253, closes #257

• [ST] Create a unit-test seam around transaction submission #259 — Added transaction-submitter.ts: TransactionSubmitter interface + MockSubmitter so tests can inject a fake Stellar layer without hitting Horizon.
• [ST] Define a payout failure reason taxonomy #256 — Added payout-failure-taxonomy.ts: typed PayoutFailureCode enum, severity levels, and classifyPayoutError helper for debugging failed settlements.
• [ST] Write an escrow state-machine contract document #253 — Added escrow-state-machine.ts: full state/transition table for queue buddy escrow lifecycle (CREATED → FUNDED → … → RELEASED/REFUNDED).
• [ST] Improve the Stellar contributor README #257 — Added smoke-test.ts: runnable contributor onboarding script that checks Horizon reachability, account existence, and dry-runs a tx build.

Shared EscrowState and PayoutFailureCode types also exported from libs/types.