docs(5243): spec, plan, research, tasks for ledger minting
Smallest first slice toward unblocking issue #5285 Story 1: plumb a mint set through buildLedgerTx and buildLedgerTxRaw, translate the wallet's (TokenMap, TokenMap) mint/burn pair to a single signed ledger MultiAsset, and feed the real value from mkTransactionLedger and constructUnsignedTxLedger. Script witnesses remain out of scope. Artifacts: spec.md (2 P1 stories, 9 FRs, 5 SCs), plan.md (113 lines, constitution pass, 6 design decisions), research.md (Phase 0 from Explore agent: signed/unsigned asymmetry between TokenMap and MultiAsset is the load-bearing surprise), data-model.md (netting rule for mint/burn overlap), contracts/ledger-mint-translation.md (P1-P8 converter properties, B1-B3 builder properties), tasks.md (15 tasks across 5 phases mapping to three bisect-safe commits), requirements checklist, quickstart for reviewers.