fix(runtime): epic train follow-ups (round 2)

[mfw78]

What

Two deferred M0 follow-ups from the epic train, one commit each.

1. Typed BuildError for the component build path. ComponentsBuilder::build now returns Result<Components<T>, BuildError>, where BuildError names the failing slot (Chain, Store, or Ext) instead of an opaque anyhow::Error. The leaf ComponentBuilder::build stays on anyhow because the backends fail for heterogeneous reasons (I/O for the store, network for the chain), so a single typed cause there would be dishonest. Callers that propagate into anyhow keep working through the std::error::Error conversion thiserror derives.
2. Explicit executor seam at the CLI launch root. run_from_config now passes .with_executor(&TokioExecutor) in the builder chain rather than relying on the builder's implicit tokio default. This names the launch root as the point where an embedder or a non-tokio target substitutes its own executor. Behaviour-preserving: the binary still spawns on tokio.

Why

Both were recognized during the epic train review as realizable clean-ups that were deferred to keep that PR focused. The typed error makes a build failure self-describing at the boundary; the explicit executor call turns an implicit default into a decision made where the launch is composed. Neither changes runtime behaviour.

Testing

• cargo clippy -p nexum-runtime -p nexum-cli --all-targets clean, zero warnings (pinned nix toolchain, rustc 1.94.0).
• cargo test -p nexum-runtime host::component: 6 passed.
• cargo test -p nexum-runtime runtime::task: 4 passed.
• rustfmt --check clean on all touched files.

AI Assistance

AI Assistance: Claude Code (Opus 4.8) used for implementing both commits, verification, and drafting this PR body.

[mfw78]

Superseded by the consolidated follow-up PR #278, which carries this change as a preserved commit. Closing to reduce the outstanding-PR count; the commit history is unchanged.