docs(spec): design for SP27 remittances architecture refactor
Tier 1: split scheduler.py into handlers/ subpackage, dedup helpers,
loosen INBOUND_RE, add SFTP operation timeouts, surface SFTP errors
in Scheduler.status().
Tier 2: unify 835 ingest + reconciliation into one critical section,
add GET /api/claims/{id}/chain, guard matched_remittance_id ↔
Remittance.claim_id invariant, emit claim.rejected_after_remit audit
when a 277CA rejection hits a matched claim.
Status: Draft, awaiting user sign-off.
Nora
454c3598b1