Implement ThreadPoolExecutor core

[enricopiovesan]

Summary

• Add ThreadPoolExecutor, ThreadPoolExecutorConfig, and ConfigError in traverse-runtime.
• Dispatch native capability execution onto a bounded Rayon pool.
• Reject WASM artifacts before pool dispatch and catch native panics as ExecutorError::ExecutionFailed.
• Add focused coverage for constructor validation, success, error propagation, WASM rejection, panic mapping, debug, and config error formatting.

Governing Spec

• 001-foundation-v0-1
• 006-runtime-request-execution
• 007-workflow-registry-traversal
• 010-runtime-state-machine
• 012-execution-trace-tiered
• 013-browser-runtime-subscription
• 016-runtime-placement-router
• 017-ai-agent-packaging
• 018-event-driven-composition
• 024-placement-constraint-evaluator
• 025-wasm-executor-adapter
• 026-event-broker
• 029-integrated-observability
• 030-security-identity-model
• 031-supply-chain-hardening
• 032-universal-data-access
• 033-http-json-api
• 034-programmatic-registration
• 035-multi-agent-isolation
• 036-event-subscription-replay
• 037-semver-range-resolution
• 038-wasi-host-insulation
• 040-contractual-enforcement-gate
• 041-workflow-composition-api
• 043-module-dependency-management
• 044-application-bundle-manifest
• 045-governed-model-dependency-resolution
• 046-public-cli-app-registration
• 047-thread-pool-executor
• 048-semver-publishing-pipeline
• 049-v1-milestone-gate

Project Item

• Closes Implement ThreadPoolExecutor core (spec 047) #505
• Project 1 item: PVTI_lADOEbiBt84Bbyp1zgxqu_Q

Validation

• cargo test -p traverse-runtime
• cargo clippy -p traverse-runtime -- -D warnings
• bash scripts/ci/repository_checks.sh
• PATH=/opt/homebrew/opt/rustup/bin:$HOME/.cargo/bin:$PATH cargo build --target wasm32-wasip1 -p traverse-expedition-wasm
• PATH="$HOME/.cargo/bin:$PATH" LLVM_COV=/Users/enricopiovesan/.rustup/toolchains/1.94.0-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/bin/llvm-cov LLVM_PROFDATA=/Users/enricopiovesan/.rustup/toolchains/1.94.0-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/bin/llvm-profdata bash scripts/ci/coverage_gate.sh
• git diff --check