docs: add docs-as-code process and one ADR per feature

docs/docs-as-code.md defines the workflow; docs/adr holds 31 ADRs
(retroactive, reconstructed from code and changelog) with a template and
index. docs/check-docs.sh validates ADR structure, index, cited code
paths, relative links and CLAUDE.md/AGENTS.md sync.

docs/adr/0009-custom-electrum-client.md

@@ -0,0 +1,24 @@
+# 0009. Hand-written Electrum JSON-RPC client with pipelining and an in-flight cap
+
+- **Status:** Accepted
+- **Area:** Network
+- **Recorded:** 2026-11-03 (retroactive: reconstructed from code, comments and changelog)
+
+## Context
+
+The wallet talks to an ElectrumX/Fulcrum-style indexing server (ports 50001/50002, not the node's P2P port 2333). No library matched the needs: protocol 1.4, a testable in-process transport and control over throughput on large wallets.
+
+## Decision
+
+Implement `ElectrumClient` directly over `TcpClient`/`SslStream`, newline-delimited JSON-RPC 2.0. A single-reader channel batches writes (like Electrum's buffered writer); a read loop completes pending requests by id; a semaphore caps in-flight requests (`DefaultMaxInFlight = 32`, configurable per connection) to avoid server throttling and disconnects. Typed helpers live in `ElectrumApi`. The class is deliberately not an interface: tests run against a real in-process server instead (0025).
+
+## Consequences
+
+- Full control over request pipelining and error mapping (`ElectrumServerException`).
+- No single in-flight value suits every server, hence the per-connection knob.
+- Protocol quirks (e.g. `server.version` allowed once per connection) are our responsibility; the server probe (0027) exists to catch server differences.
+
+## References
+
+- `src/Core/Net/ElectrumClient.cs`
+- `src/Core/Net/ElectrumApi.cs`