feat(bindings): execute native phases from R and MATLAB

[GBeurier]

Summary

• add dagml_execution_plan_execute_phase_json to execute one ExecutionPlan phase through the native scheduler with trusted manifest validation before callbacks
• expose R and MATLAB/Octave wrappers that dispatch exact NodeTask JSON to process-local callbacks and return core-validated NodeResult JSON
• document the borrowed-vtable lifecycle contract and refresh ABI/criteria/training/replay contract packs

Validation

• cargo fmt --all --check
• cargo test -p dag-ml-capi
• cargo clippy -p dag-ml-capi --all-targets -- -D warnings
• python3 scripts/validate_contracts.py
• python3 scripts/validate_training_replay_contracts.py
• python3 scripts/check_so_freshness.py
• git diff --check

Local gaps

• R CMD build was not runnable locally: R is not installed in this environment
• mkoctfile MEX builds were not runnable locally: mkoctfile is not installed in this environment