feat(plugins): add read-only multi-instance E64 sensor

[MaxRink]

Summary

Add an opt-in E64 sensor plugin that reads grinder telemetry through the existing host-owned WebSocket transport, with independent same-model instances and no grinder writes.

• Keep one registration, socket, request counter, pending map, timer, and connection epoch per configured instance.
• Expose only readState, readConfig, readMachineInfo, and readLogMessages, with exact request/ref correlation and complete typed object publications.
• Validate configuration and secure tokens before opening sockets, isolate one instance's failure, and preserve existing brewing/dosing scale registrations.

Linked Issue

Related #854. Depends on the sensor-selection fix #847 for accepted issue #831. #747/#748 concern separate MOTTO80 work and are context only, not issues fixed by this E64 plugin.

The new discussion issue is awaiting maintainer triage. This PR remains a draft while maintainers discuss the proposal; opening the issue does not satisfy the accepted-issue gate or imply approval of the proposed contract.

Verification

• test/plugins/e64ws_plugin_test.dart has 12 passing protocol and loopback tests covering the four reads, correlation, validation, limits, failures, polling, reconnect, heartbeat, and timeouts.

• test/controllers/steam_sequencer_test.dart has 19 passing tests, including the mixed-adapter case with two E64 adapters, two scales, and a Bengle milk probe in both registration orders, probe attach, and reconnect.

• The standalone E64 branch Flutter suite passed 4,171 tests; the final combined feature suite, including fix(api): resolve encoded sensor IDs in REST and WebSocket routes #858, passed 4,293 tests. Each had 1 skipped and 0 failures; flutter analyze is clean. Node syntax checking and Dart formatting pass locally.

• The final two-fixture runtime run passed using percent-encoded IDs for REST manifest lookup, all four read commands, and independent WebSocket snapshots. Disconnecting A leaves B responsive and the simulated scale connected. This uses the host fix in fix(api): resolve encoded sensor IDs in REST and WebSocket routes #858. Real hardware and self-signed WSS validation are not claimed.

• The implementation and loopback fixtures were independently authored; no vendor code or assets are included.

• The final two-instance runtime check exposed pre-existing sensor path handling bug fix(api): resolve URL-encoded sensor IDs in REST and WebSocket routes #857: URL-encoded plugin sensor IDs return 404 despite both registrations being present. Draft fix(api): resolve encoded sensor IDs in REST and WebSocket routes #858 supplies that host fix and must land before this plugin is considered ready; its standalone full suite passed 4,155 tests. The final combined encoded-path runtime check now passes with that fix.

Impact

This adds an opt-in telemetry-only sensor path. It sends no motor, calibration, brew-event, or GBS writes, and it does not alter the existing scale API or inventory contract. The prerequisite sensor-selection fix is included in this branch and must land first through #847.

Contributor Responsibility

AI-assisted development is allowed. The submitter remains responsible for the submitted work.

• I have reviewed and understand all changes in this PR and take responsibility for their correctness, security, behavior, licensing, and provenance, including any AI-assisted or AI-generated work.