feat(runtime): thread real emitted events through LocalExecutor, retire output-JSON convention

[enricopiovesan]

Summary

LocalExecutor::execute returned a bare Value, discarding any events a
native or WASM-backed capability emitted during workflow-internal node
execution (ArtifactRouter, used directly by workflows.rs) and hardcoding
emitted_events: Vec::new() on the live PlacementRouter path
(BoundLocalExecutor). workflows.rs also read a JSON "emitted_events"
convention key out of the node output, rather than a structured field.

This PR extends LocalExecutor::execute to return
LocalExecutionOutput { value, emitted_events: Vec<TraverseEvent> }, threads
real WASM-sourced events through both call paths, validates natively-emitted
events against the capability's emits declaration and Subscribable
service_type before they're used for anything (mirroring
PlacementRouter Step 5's WASM-boundary validation), and publishes
workflow-node events to EventBroker for Subscribable capabilities
(best-effort — a publish failure does not fail the workflow step). The old
JSON "emitted_events" convention key is removed. All ~21 LocalExecutor
implementors across traverse-runtime, traverse-mcp, and traverse-cli
are migrated to the new return type.

Governing Spec

• 101-local-executor-event-emission
• 518-durable-local-datastore
• 006-runtime-request-execution
• 001-foundation-v0-1

101-local-executor-event-emission (ADR-0037) is the primary spec; it
governs crates/traverse-runtime/src/{lib,artifact_router,workflows}.rs.
518-durable-local-datastore covers crates/traverse-runtime/tests/.
006-runtime-request-execution covers
crates/traverse-runtime/examples/load_workspace_app_state.rs.
001-foundation-v0-1 covers crates/traverse-cli/ and
crates/traverse-mcp/ (mechanical LocalExecutor return-type migration
only; no behavioral change to those crates).

Changes

• LocalExecutionOutput { value, emitted_events } replaces the bare Value
  return type on LocalExecutor::execute.
• BoundLocalExecutor (native PlacementRouter path) validates natively
  emitted events against emits/Subscribable before returning them in
  ExecutorOutput, so PlacementRouter Step 5 publishes real events instead
  of the previous hardcoded empty vec.
• ArtifactRouter::execute returns the real emitted_events from
  WasmExecutor (already ABI-validated) instead of discarding them. It does
  not hold an EventBroker or publish internally — it's used both directly
  by workflows.rs and, via BoundLocalExecutor, by PlacementRouter, so
  publishing there would double-publish on the live Runtime::execute()
  path.
• workflows.rs's traverse_workflow validates natively-emitted events the
  same way, publishes to EventBroker for Subscribable capabilities
  (best-effort), and reads emitted events from
  LocalExecutionOutput.emitted_events instead of parsing a JSON
  "emitted_events" key out of the node output. The JSON-convention parser
  function is deleted.
• All LocalExecutor implementors (production and test doubles) across the
  workspace updated to the new return type.

Validation

• cargo test --workspace — all suites green (traverse-runtime lib: 293
  tests, including 3 new BoundLocalExecutor/PlacementRouter tests for
  declared/undeclared/non-Subscribable native events, 2 new
  workflow-node → EventBroker publish tests, and a new ArtifactRouter
  real-WASM-success test)
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --all --check — clean
• bash scripts/ci/coverage_gate.sh — passed;
  traverse-runtime: 100.00% (19510/19510 lines),
  traverse-contracts: 100.00%, traverse-embedder: 100.00%,
  traverse-mcp: 98.89%, traverse-cli-rs: 87.16%,
  traverse-swift-host: 84.03% (all at or above their configured floor)
• BASE_SHA=origin/main bash scripts/ci/local_preflight.sh — pending

[enricopiovesan]

Superseded by #1024, which implemented and merged this exact scope (LocalExecutor event emission across the same 11 files) on 2026-08-09, before this PR was ready. Issue #996 and its Project 1 item are already Done. Closing without merging to avoid duplicate/conflicting work.