docs: architectural docs for tx-generator component + cardano_node_tx_generator testnet
Two new docs pages: - docs/components/tx-generator.md — describes the daemon's role (composer-as-clock, deterministic per-request, monotonic population growth), the in-process N2C topology (single bearer for ChainSync + LSQ + LTxS, in-tree address-to-UTxO indexer), the NDJSON wire schema with response classes (Ok / IndexNotReady / NoPickableSource / SubmitRejected), the per-request build → pre-submit probe → submit → post-submit recovery flow, the reconnect-resilience PR stack (#105/#110/#114/#115/#116/#117/#118) and what each PR contributes, the composer scripts and the always-exit-0 + set -u rule, persistent state on disk, and the hard-failure vs reachability assertion classes. - docs/testnets/cardano-node-tx-generator.md — sibling testnet rationale (master is read-only on feature branches), image-set parity table with master, network topology mirror (3-pool + 2-relay; tx-generator → relay1 over single N2C bearer), local-run + Antithesis-dispatch invocations, the sibling publish-images workflow, and the master-promotion pattern. mkdocs.yml updated with both pages in the Components and Testnets nav sections respectively.