feat(fuml): translate classes, structural features, signals and active-class behavior in the referee's emitter

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What and why

The fUML referee's emitter (tools/referee/fuml/emit.go) translated only the primitive-valued activities of the reference implementation's test models; nine expressible activities stayed not-expressible with a not yet translated: reason because they create objects, write structural features, accept signals or run a class's behavior. This PR grows the emitter over those constructs, one construct group per commit, and extends the referee so object-valued inputs and outputs execute and compare.

Draft — work in progress. The status of each group:

Done: classes, object creation, structural features

• fUML Class → part def, generals as :> supertypes, each attribute at its exact declared multiplicity ([0..*] ordered nonunique, [0..3], …; the UML default [1..1] unwritten). Classes are resolved by ID first and then by name, so a type reference with a usable ID but no name resolves.
• CreateObjectAction → action N { out result : T = new T(); } — creation starts no behavior (that is StartObjectBehaviorAction, under active classes below).
• ReadStructuralFeatureAction → out result … = object.f; at the feature's multiplicity.
• AddStructuralFeatureValueAction / RemoveStructuralFeatureValueAction / ClearStructuralFeatureAction → assign object.f := …; with the reference implementation's semantics: a replacing add or a single-valued feature takes the value; a unique feature drops its old copy; an unpositioned add inserts first (the reference's FirstChoiceStrategy), insertAt inserts at that position with * appending; a remove drops every copy (isRemoveDuplicates), the copy at removeAt, or the first copy; clear empties. The original object flows on through the result pin.
• ReadSelfAction → this inside a class-owned behavior; at the top level it stays a typed TranslateError ("reads self in an activity no class owns").
• Untyped parameters (the reference runs them as String) are declared without a type; an inout parameter's input and output nodes are told apart by their edges.
• The referee (tools/referee/fuml/run.go) now materializes class-typed inputs the way the reference's Environment.makeValue does (an object per class, features defaulted recursively) and renders object outputs by type and feature, numbered by first mention, so Object/Reference records compare.

Done: signals

• fUML Signal → attribute def with its attributes, specializing its generals, declared before the classes and activities of the closure.
• SendSignalAction → an action sending new <Signal>(…) to the object at its target pin, one argument pin per attribute, inherited ones included.
• AcceptEventAction of one SignalEvent → an accept node whose result pin is the instance received; a specialized signal satisfies an accept of its general through the runtime's conformance matching.
• Runtime: the lowering (internal/ir/lower/action_graph.go) records an accept's payload parameter as the node's output feature and the executor (internal/exec/runtime/action_executor.go) binds the accepted value on the accept's own performance, so an object flow out of the accept pin carries the instance on. Conformance pair accept_payload_flows_from_pin and TestRuntimeRobustnessAcceptPayload cover it.
• No row moves yet: the suite's two signal activities also use an activity as a class (TestSignalReceiver is a top-level activity with an attribute that TestSpecializedSignalSend instantiates and starts), so their reasons sharpen and their buckets stay until the active-class group lands.

Merged: develop's layering relayout

origin/develop is merged in (internal/core/runtime → internal/exec/runtime, internal/core/lower → internal/ir/lower, corpus gates → tests/corpus). Every file this PR adds or edits is at its new path; the runtime's added conformance pair and robustness test were relocated by hand. The relayout changed nothing in substance for this PR — only paths.

Referee so far (go run -C tools ./cmd/fuml-referee -jobs 8):

bucket	before	after
pass	15	20
fail	0	0
not-expressible	36	31
differs-by-design	4	4

Moved to pass: TestGeneralizationAssembly, TestClassObjectCreator, TestClassWriterReader, TestClassAttributeWriter, TestClassAttributeValueRemover, adjudicated in docs/project/fuml-referee.md; the baseline is updated and -check is clean. The classifier's 27 rows did not move. Four rows remain the emitter's: ActiveClassBehavior, ActiveClassBehaviorSender, TestSignalReceiver, TestSpecializedSignalSend.

Remaining

• Active classes: a class's owned behavior emitted as the part def's performed/exhibited behavior, StartObjectBehaviorAction starting it asynchronously (never creation), an activity used as a class (ActiveClassBehavior, ActiveClassBehaviorSender, TestSignalReceiver, TestSpecializedSignalSend).
• Edge weights and object-flow cycles through control nodes, if the runtime supports them; otherwise a fail with the reason.
• Final docs/project/fuml-referee.md / spec-compliance.md / changelog pass, -update / -check, -json determinism, full gates, un-draft.

Specification basis

fUML 1.5 (formal/2021-07-01) § 8.6 Actions — CreateObjectAction, ReadSelfAction, the structural feature actions, SendSignalAction, AcceptEventAction — as the pinned reference implementation (v1.5.0a) executes them; § 8.8 for the object-behavior startup still to come. The spec-compliance.md row citing the fUML referee carries the counts above.

How it was verified

• tools/referee/fuml/emit_test.go: TestEmitClasses, TestEmitObjectCreationAndFeatureWrites, TestEmitFeatureReads, TestEmitReadSelf, TestEmitRefusesAnActivityAsObject, TestExecuteObjects, TestEmitSignals, TestEmitSignalTypedAttribute, TestEmitSendSignal, TestEmitAcceptEvent, TestExecuteSignals on hand-built models; every emitted model passes Validate.
• internal/exec/runtime: conformance pair testdata/conformance/accept_payload_flows_from_pin.{sysml,expected.json}, robustness_accept_payload_test.go.
• go build ./..., go vet ./..., gofmt -l . (empty), go test ./... (root and tools/), make lint, make docs-check, python3 scripts/changelog.py check, mkdocs build --strict — all clean after the merge.
• OPENSYSML_REQUIRE_TRAINING_CORPUS=1 OPENSYSML_REQUIRE_PILOT_CORPORA=1 go test -count=1 ./tests/corpus -run 'TestTrainingExamples|TestPilotCorpora' clean.
• ./scripts/download-fuml-suite.sh && go run -C tools ./cmd/fuml-referee -jobs 8 -check clean; -json twice byte-identical; the baseline's provenance (tag, commit, digests) unchanged.

Checklist

• make test and make lint pass locally
• Tests added or updated for the change
• Documentation extended where it already covers the surface (see CONTRIBUTING.md)
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved (compliance rows need nothing: the census is counted at docs build)
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

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