feat(runtime): execute terminate in actions

[devin-ai-integration]

What and why

terminate written in an action had two silent no-op paths: action stop terminate; lost its keyword at parse time and ran as an empty action, and lower.lowerBody dropped a terminate; / terminate c1; written in a nested action node's body. This PR makes terminate execute in actions.

Actions only. A terminate in a state's entry/do/exit body, a transition to a terminate action usage (accept Abort then stop; action stop terminate;), and terminate <expr> naming an occurrence instance (terminate this;) are not part of this change; they keep typed refusals ('terminate' in a body is not executable, ErrTerminateOccurrence) and are marked ❌ in docs/project/spec-compliance.md.

Parser, AST, lowering, export

• ast.Usage.IsTerminate marks a terminate action usage; the parser records it instead of dropping the keyword, and no longer reads a keyword after an action node's name as an action reference. Dumped in ast/dump.go, carried by the AST codec (snapshot format bumped to 17 so a pre-change blob is refused as stale; embedded stdlib snapshot regenerated). The keyword is accepted on action usages only; part p terminate; is diagnosed.
• lower.lowerBody includes *ast.TerminateStatement; a terminate action usage lowers to a one-statement body. lower.Effect carries the resolved target so the executor re-resolves nothing:

  type Effect struct {
      Kind       EffectKind
      Node       ast.Node
      Scope      *symbols.Scope
      Terminates TerminateTarget // Containing | Enclosing | Node | Occurrence | Unknown
      Target     ast.Node        // the action node, for TerminateNode
      TargetExpr ast.Node
  }

  TerminateContaining is a bare terminate;; TerminateEnclosing is a terminate action usage (it ends the flow it is a step of, not its own one-statement performance); TerminateNode names an action node; TerminateOccurrence is a feature or expression that is not an action node.
• RDF mapping: a terminate action usage is written as sysml:TerminateActionUsage and read back as action <name> terminate;.

Executor (internal/core/runtime/action_terminate.go)

• actionStmtHost.effect dispatches EffectTerminate to performances.terminate, which resolves the target performance (terminateTarget: the body's own frame, its parent, or the named node's frame found as f.node or in f.subactions walking up the frame chain) and either unwinds the body with a typed *terminated{perf} error when the body runs within the target, or ends the target in place (endOther) when it is a sibling flow.
• stepToken catches the unwinding (endTerminatedFor) at the token whose step is the outermost within the target and ends it (endAround): every other token inFlowOf the performance is dropped lowest ID first (dropTokensIn) — paused bodyRuns ended, nested frames marked ended — and recorded as terminate <perf>: dropped token N@node, ...; the surviving token leaves the node (leaveTerminated) through endPerformance + completeNode, so the parent takes the node's succession with the outputs assigned so far and no inner succession fires. The root ends with StateCompleted and endPerformanceLife.
• A statement-driven nested node (performNode) and a body-driven flow (usageWork.perform, runSubflow) treat a terminate of their own performance as completion.
• New sentinels: ErrTerminateTarget (name is no action node of a flow around the statement, or nothing runs in it), ErrPerformanceEnded (target already ended), ErrTerminateOccurrence (the out-of-scope occurrence target). Calculations still refuse with ErrCalcSideEffect; an unsequenced terminate among an action's members is still ErrStatementOutsideFlow.

Docs

• docs/project/spec-compliance.md: Actions map gains rows for the flow node, the named terminate action usage, the nil-target body statement, the named-node body statement, occurrence targets (❌) and state positions (❌); the Track E paragraph drops the action clause only.
• docs/guide/06-behavior.md: a terminate paragraph with the action-only scope.
• changes/unreleased/terminate-action-execution.added.md.

Known limitations

• The training example 19. Terminate Actions/Terminate Actions Example-1.sysml still stops at no initial node found in action node performCriticalActivity: its body has no first/start. That is the flow-start rule, unchanged here; terminate is proven on fixtures that declare a start.
• terminate <expr> on an occurrence, and every state-machine position, are typed refusals pending the state-machine half.

Specification basis

SysML v2 §7.17.10 (terminate action usages): a terminate with no target ends the immediately containing performance; a named target ends that action node's ongoing performance; the parent continues along the node's succession.

How it was verified

• go build ./..., go vet ./..., gofmt -l . (empty), go test ./..., go test -race ./..., make lint: all clean.
• go test -run TestGolden ./internal/core/parser — new fixture action_terminate.sysml (then terminate;, action stop terminate;, then action a2 terminate;, terminate c1; and terminate; in a body); no other golden moved. TestNegative gains part_terminate, attribute_terminate, state_terminate.
• go test -run TestStdlibConformance ./internal/core/libs.
• go test -run 'TestExecutionConformance|TestExecutionTrace' ./internal/core/runtime — nine new fixtures under testdata/conformance/action_terminate_*: flow node, nested body, named usage, named usage reached over two successions that synchronize (trace golden), fork dropping a sibling (trace golden), nested fork (trace golden), naming the node from its own body, naming the enclosing node, naming a sibling flow's node (trace golden).
• go test -run TestRuntimeRobustness ./internal/core/runtime — terminate_of_an_ended_performance, terminate_of_an_unknown_name, terminate_of_a_non_action_feature, terminate_of_an_occurrence_expression, terminate_of_a_node_of_a_sibling_flow; calc_terminate_is_rejected unchanged.
• OPENSYSML_REQUIRE_TRAINING_CORPUS=1 OPENSYSML_REQUIRE_PILOT_CORPORA=1 go test -count=1 ./internal/core/model -run 'TestTrainingExamples|TestPilotCorpora' — ok; training_examples_expected.txt unchanged.
• OPENSYSML_REQUIRE_TRAINING_CORPUS=1 OPENSYSML_REQUIRE_PILOT_CORPORA=1 go test -count=1 ./internal/core/export -run TestCorpusRoundTrip — ok, no file moved.
• make docs-counts, make docs-check, python3 scripts/changelog.py check — ok.

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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