fix(cli): pass durable System to context-based class exporters

Summary

r2x run <exporter> -i <system.json> rejects a context-based class exporter even after the bridge has loaded the System into PluginContext.system.

This blocks the public durable-boundary contract for r2x-plexos.plexos-exporter.

Environment

• r2x-cli 0.1.8 (0d384a15cb96c360afb1ff8cc126263f6021d02f)
• r2x-plexos 0.3.0 (51a93284ddeffa3a0964c780cd3c5565b9bd6e8f)
• r2x sync completed successfully and does not change the result.

Reproducer

After creating a valid durable translated System with the public CLI:

r2x run r2x-plexos.plexos-exporter \
  -i /tmp/geothermal-r2x-018-short-smoke/translated-system.json \
  --output-path /tmp/geothermal-exporter-repro \
  --model-name Geothermal_A1 \
  --horizon-year 2050 \
  --weather-year 2012 \
  --template PLEXOS10.0

Verbose output shows the contradictory dispatch:

DEBUG: Invoking plugin with target: r2x_plexos:PLEXOSExporter.export
DEBUG: Parsing target: r2x_plexos:PLEXOSExporter.export
DEBUG: Using method 'run' for plugin 'PLEXOSExporter'
Error: Run command failed: Python bridge error: Plugin stream error: plugin method 'PLEXOSExporter.run' does not accept a System or JSON input

The command then aborts with the known GIL finalization failure.

Why this is incorrect

PLEXOSExporter is a context-based r2x_core.Plugin:

• It implements on_export(self) and obtains the model through self.system.
• self.system reads PluginContext.system.
• It inherits Plugin.run(self, *, ctx=None); it intentionally accepts no positional System argument.

The bridge already deserializes the -i artifact and passes it into PluginContext.system for the exporter role. It then chooses inherited run() and applies the generic positional-System signature check, rejecting the input before run() can execute on_export().

Additionally, the role-derived PLEXOSExporter.export call target is synthetic: this exporter defines no concrete export() method.

Expected behavior

For a context-based exporter:

1. Deserialize -i into PluginContext.system.
2. Construct the plugin from that context.
3. Invoke its lifecycle run() with no positional payload.
4. Treat the exporter as a terminal sink.

Suggested fix

In the class-plugin bridge, exempt context-based exporter lifecycle invocation from the generic System/JSON positional-argument validation. Once the bridge has populated PluginContext.system, call run() with no positional argument.

Avoid treating a role-derived Class.export string as a concrete class method unless the plugin defines that method. An adapter convention is also viable, but it should preserve the documented r2x run <exporter> -i <system.json> contract.

Regression coverage

Add an integration fixture that:

• defines a class exporter inheriting Plugin, with on_export() reading self.system and no concrete export() or run() override;
• invokes it through direct short-form r2x run <plugin-ref> -i <durable-system.json>;
• verifies the exporter receives the System and writes its configured terminal artifact;
• verifies no JSON is emitted to stdout.

This should cover real r2x-plexos exporter lifecycle semantics without requiring PLEXOS in the CLI test suite.

Start at the class-plugin bridge used by the direct short-form r2x run <plugin-ref> -i <durable-system.json> entry point, then reproduce the exporter failure described above. Add the integration fixture with a Plugin exporter whose on_export() reads self.system and has no concrete export() or run() override. Done means the System reaches the exporter, its terminal artifact is written, and no JSON is emitted to stdout.