TransactionReader adapter

[karthikiyer56]

What

A concrete type in internal/rpcv2/adapters satisfying store.TransactionReader (one method, GetTransaction), backed by the query router's by-hash probe surface (#865, implemented in PR #894; design in PR #843): probe the hot transaction-hash indexes first, then each window's frozen index, verify candidates against the full hash, and gate the result to the read view's [floor, latest] range.

The probe algorithm already exists in stores/txhash/read_assembly.go — this ticket wires it up and gates it.

What the read view provides

Call	File	What it does for us
view.HotTxHashIndexes()	query/txlookup.go	[]txhash.HashIndex for every published hot chunk, newest first. Ready to pass straight into NewTxReader. Registry-owned — do not close.
view.ColdTxHashIndexCoverages()	query/txlookup.go	Frozen coverage keys, newest Hi first, read through the snapshot so a concurrent index rebuild can't change the probe set mid-request.

What this ticket owns

• Open the cold .idx files. Coverages are keys, not readers. Per coverage: txhash.OpenColdReader(layout.TxHashIndexFilePath(cov)). These are not view-owned — we close them. (Unless we add ColdTxIndexes() — see the open decision below.)
• txhash.LedgerSource — one method, GetLedgerRaw(seq). Plain per-candidate resolution: view.Ledgers(chunk.IDFromLedger(seq)), one open per candidate. No caching layer: TxReader.scan takes one candidate per index, index ranges are disjoint, and the probe stops at the first verified match, so no chunk is ever probed twice.
• The floor/latest gate. HotTxHashIndexes is deliberately unfiltered (its doc comment says so): a hot handle can predate the view's floor, and a cold index hit is a fingerprint, not an exact match. A resolved ledger outside [OldestLedger(), LatestLedger()] must become store.ErrNoTransaction, not an error and not a served result.
• ingest.LedgerTransactionView → store.Transaction. Near 1:1 field copy: Hash, Result, Meta, Envelope, DiagnosticEvents, TransactionEvents, ContractEvents, FeeBump, ApplicationOrder, Successful, LedgerSequence, LedgerCloseTime all land directly. Only the hash needs hex encoding and the ledger fields need wrapping in store.LedgerInfo.
• Preserve the soft-error contract. TxReader.GetTransaction deliberately returns an error rather than a clean not-found when a candidate could not be verified. Do not collapse that into ErrNoTransaction — a false not-found is indistinguishable from the transaction genuinely not existing.

Open decision for this ticket

The #894 reviewer proposed a ColdTxIndexes() ([]txhash.HashIndex, error) on ReadView that opens each frozen coverage newest-first with view-owned closers, feeding NewTxReader's slice directly and letting ColdTxHashIndexCoverages go internal. Deferred to this ticket, where it gains its consumer. Recommendation: add it. Without it this adapter is the only cold-reader consumer holding its own path composition and closer discipline.

Sizing

~120–180 lines in 1 file, ~12 tests.

Depends on

#876, #865 (implemented in PR #894). #885 in build order only — nothing is code-shared; the LedgerSource is a plain per-candidate view.Ledgers read.