Add opt-in current and fault telemetry

[BanjoR]

Authoritative references

• Upstream reference: DCC-EX/CommandStation-EX#260, the stale/incomplete current-measurement proposal being refreshed.
• Superseded fork-only publication: BanjoR/CommandStation-EX#3.

Bounded scope

This refresh changes exactly five source files:

• CommandDistributor.cpp / CommandDistributor.h: add a client-safe current/fault broadcast using the existing CommandDistributor path.
• DCCEXParser.cpp: add opt-in <JI 1> and <JI 0> controls while preserving the existing plain <JI> query.
• TrackManager.cpp / TrackManager.h: add disabled-by-default, 100 ms-throttled telemetry from existing current and overload/fault state.

When enabled, events use <jI TRACK CURRENT_MA FAULT> with zero-based track number, measured milliamps, and 0/1 fault state. The implementation is opt-in and broadcasts through the client-safe distributor rather than retaining the parser's transient Wi-Fi/Ethernet stream.

Explicit exclusions:

• No change to the established plain <JI> response.
• No new <A> or <a> protocol command and no internal RMS/history-buffer design.
• No motor-shield calibration changes.
• No global diagnostic default changes or GPIO instrumentation.
• No hardware, vendor, protocol, integration, or unrelated subsystem changes.

Validation

All results below refer to head commit 37cda773.

• PASS — git diff --check.
• PASS — clean-worktree, live upstream-ancestry, five-file allowlist, line-ending, generated-artifact, and forbidden-change checks.
• PASS — focused host assertions covering <JI> backward compatibility, <JI 1>/<JI 0> gating, <jI> event formatting, default-disabled state, 100 ms throttling, current/fault sources, and safe broadcast routing.
• PASS - the exact default PlatformIO matrix on this branch completed successfully for all five configured environments: mega2560, ESP32, Nucleo-F411RE, Nucleo-F446RE, and Nucleo-F429ZI.
• CI unavailable in the upstream PR context because the firmware workflow is push-triggered; the exact local five-environment matrix above passed. Ancillary Docs and Label runs are not firmware validation results.
• Host-side Ztest execution is not available in this repository; the exact firmware matrix above passed, but hardware/runtime coverage remains maintainer-only.

Hardware validation

Not run—no hardware available.

Maintainer bench criteria:

1. Build and flash the validated firmware with a supported motor shield and a configured current-sense/fault-pin setup.
2. Connect a command client, issue <JI 1>, and verify nominal approximately-100-ms <jI TRACK CURRENT_MA 0> events for every active track, with the reported milliamps matching an independent meter or known load within the board's expected accuracy.
3. Trigger a supported fault-pin or overload condition and verify the affected track reports <jI TRACK CURRENT_MA 1> and enters the existing overload state; clear the condition and verify recovery behavior.
4. Issue <JI 0> and verify asynchronous events stop, while a plain <JI> still returns the established one-shot current response.
5. Repeat with Wi-Fi/Ethernet clients and multiple clients to confirm broadcasts do not corrupt or cross-contaminate client buffers.

This pull request is ready for maintainer review; hardware validation remains outstanding; local firmware compilation is complete.

Current exact-head CI status

N/A — No BanjoR fork Actions run exists for exact head 37cda773ff695da09f23ffa8e551e90650266b57; no hosted code-test result is claimed. Local validation above is the available evidence.