[Rust port] Cargo workspace skeleton

[willwashburn]

Parent: #240

Context

Stand up the Cargo workspace skeleton so subsequent crate ports have a place to land. This unblocks every other sub-issue in the epic.

Scope

• Create crates/ directory at repo root with empty Cargo.tomls for the 7 crates listed in Epic: rewrite burn in Rust (v2 — supersedes #222) #240's workspace shape: relayburn-reader, -ledger, -analyze, -ingest, -sdk, -cli, -sdk-node.
• Top-level Cargo.toml workspace manifest with members = ["crates/*"] and shared dependency versions.
• relayburn-cli Cargo.toml uses [[bin]] name = "burn" path = "src/main.rs" (the crate name is relayburn-cli because burn is taken on crates.io; the binary keeps the burn invocation).
• Pin Rust toolchain (rust-toolchain.toml) — current stable.
• .cargo/config.toml if needed for cross-compilation targets.
• CI: add a cargo build --workspace + cargo test --workspace job to the existing GitHub Actions, running alongside the TS build for now.
• Document the dual-tree (packages/ TS + crates/ Rust) in CLAUDE.md and root README.md.

Out of scope

• Any actual port work. Each crate is empty (lib.rs with // TODO).
• napi-rs setup (covered in the sub-issue for relayburn-sdk-node).

Depends on

Nothing — this unblocks everything else.

Acceptance

• cargo build --workspace succeeds on a clean checkout.
• cargo test --workspace succeeds (no tests yet, but framework runs).
• CI job is green.
• cargo install --path crates/relayburn-cli produces a burn binary that runs (even if it just prints "not yet implemented").
• pnpm run build and pnpm run test still succeed unchanged.