feat(examples): add ethercat-real-bus (EK1100 + EL1008)

[patdhlk]

Summary

Adds examples/ethercat-real-bus/ — a standalone Cargo crate that drives a real Beckhoff EK1100 + EL1008 from a Linux host (Raspberry Pi the canonical target) via EthercrabBusDriver. Reads the EL1008's 8 digital input bits at 10 ms cadence and prints the byte every time it changes.

Fourth integration example in examples/; CI compiles it but does NOT run it (no NIC, no hardware in CI). Mirrors the established examples/* standalone-crate conventions (empty [workspace], [patch.crates-io] marker block, /target .gitignore, publish = false, no .runnable marker).

What this shows

• EthercrabBusDriver::new(&PDU_STORAGE, opts) — real-bus driver construction (bus-integration feature on taktora-connector-ethercat).
• Connecting → Up health handshake via Connector::subscribe_health.
• ChannelReader<u8, N> over an EthercatRouting PDO slice — same surface every other connector in this workspace exposes.
• A minimal RawByteCodec defined inline (JsonCodec can't decode raw PDI bytes). Purpose-built for this example; intentionally not promoted to taktora-connector-codec.

Files

New crate under examples/ethercat-real-bus/:

• Cargo.toml — pins taktora-* = "0.1" (and taktora-connector-core = "0.2"), enables bus-integration on taktora-connector-ethercat.
• Cargo.lock — committed (application crate).
• .gitignore — /target.
• README.md — hardware checklist, host setup, setcap recipe, run instructions, troubleshooting.
• src/main.rs — ~250 LoC: CLI (--nic, --ticks), RawByteCodec impl + 4 unit tests, bring-up wiring, health-polling item (250 ms), scan-and-print item (10 ms).

Workspace plumbing:

• examples/README.md — fourth row in the "What's here" table.
• README.md (root) — fourth row in the Integration examples table.
• .github/workflows/ci.yml — examples/ethercat-real-bus added to the examples job's rust-cache workspaces: list.

Test plan

• cargo build --release in the example crate.
• cargo test --release — 4 RawByteCodec round-trip + error tests pass.
• cargo clippy --release --all-targets -- -D warnings — clean.
• ./scripts/check-examples.sh — green; new example reports skipping run (no .runnable marker).
• cargo test --workspace --all-features -- --test-threads=1 — 394 passing, 0 failing.
• Manual smoke test on a Raspberry Pi + EK1100 + EL1008 (out-of-band; the example was designed for exactly this hardware).

Spec / design

• Design doc: docs/superpowers/specs/2026-05-21-ethercat-real-bus-design.md (local; docs/superpowers/ is gitignored).
• Plan: docs/superpowers/plans/2026-05-21-ethercat-real-bus.md.
• No new sphinx-needs requirements — this example is a downstream demo of the existing FEAT_0041 work.

[gemini-code-assist]

Code Review

This pull request introduces the ethercat-real-bus example, demonstrating real-world EtherCAT integration with Beckhoff hardware using the taktora framework. It includes a custom RawByteCodec for 8-bit digital inputs and comprehensive setup documentation for Linux hosts. Review feedback suggests optimizing state management in main.rs by removing an unnecessary Arc<Mutex> wrapper for a variable used only within a single closure, which avoids redundant synchronization and allocation overhead.

[gemini-code-assist]

The last_value state does not need to be wrapped in an Arc<Mutex> because it is only used within a single ExecutableItem closure. Since the closure is move and the executor treats items as FnMut, you can simply capture a local Option<u8> variable. This avoids unnecessary synchronization and allocation overhead.

Suggested change

	let last_value: Arc<Mutex<Option<u8>>> = Arc::new(Mutex::new(None));
	let last_value_for_item = Arc::clone(&last_value);
	let mut last_value: Option<u8> = None;

[gemini-code-assist]

Simplified usage of last_value after removing the redundant Arc<Mutex> synchronization.

                if last_value != Some(v) {
                    let elapsed_ms = started_at.elapsed().as_millis();
                    println!("t=+{elapsed_ms:>6}ms  bits=0b{v:08b}  decimal={v}");
                    last_value = Some(v);
                }