phase 1 scaffold: cargo workspace + 3 crates + roadmap + architecture

Repo skeleton for aldabra, the rust-native cardano lite wallet
with MCP server interface. Builds end-to-end, types in place,
real cardano primitives land next pass.

Crates:
  wallet-core   — pure crypto + types. mnemonic, key derivation,
                  signing. No I/O. Security boundary.
  wallet-chain  — pluggable backends. ChainBackend trait, Koios
                  client (stub for now). Ogmios + submit in phase 2.
  wallet-mcp    — the binary. stdio MCP transport via rmcp.

Phase plan in ROADMAP.md, threat model in docs/architecture.md.

This is also Sulkta's first Rust project + a real-world workout for
ci-runner's rust toolchain.

crates/wallet-chain/Cargo.toml

@@ -0,0 +1,23 @@
+# wallet-chain — pluggable backends for chain queries.
+#
+# A `ChainBackend` trait abstracts over Koios (HTTPS), Ogmios (WS/HTTP),
+# or any future option. Phase 1 ships with a Koios implementation since
+# Sulkta already runs Koios endpoints.
+#
+# Submission paths land in phase 2. Read-only queries first.
+
+[package]
+name = "wallet-chain"
+version.workspace = true
+edition.workspace = true
+license-file.workspace = true
+repository.workspace = true
+authors.workspace = true
+
+[dependencies]
+tokio = { workspace = true }
+reqwest = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+thiserror = { workspace = true }
+async-trait = "0.1"