feat(wasm): execute task-local losses through scheduler

[GBeurier]

Summary

• lower explicit native training-loss roles into execution plans with canonical per-node ordering
• execute prebuilt plans through the WASM scheduler while preserving task-local loss requirements
• require independently supplied trusted controller manifests before dispatch
• cover JavaScript-local custom losses in FIT_CV and REFIT, including stale manifests and missing attestations

Validation

• cargo fmt --all --check
• cargo test -p dag-ml-core execution_plan_lowers_training_losses_in_canonical_order
• cargo test -p dag-ml-wasm
• cargo clippy -p dag-ml-core -p dag-ml-wasm --all-targets -- -D warnings
• wasm-pack build crates/dag-ml-wasm --target nodejs --release
• independent review completed twice; final review reported no findings

Stack

• base: feat(wasm): bind task-local training losses #25 (agent/loss-cabi-controller-binding)
• matching dag-ml-data peer branch contains no data changes

Node is unavailable in the local environment; the Node 20/22 smoke is delegated to CI.