fix(npm): stream filtered output before exit

[maxjustships]

Summary

• stream npm/npx output through RTK's existing line-filter pipeline instead of retaining stdout until child exit
• preserve npm boilerplate/warning/progress filtering and the existing ok result when every line is filtered
• add an integration regression that proves readiness stdout and meaningful stderr are observable while the npm script is still alive

Why

rtk npm used FilterMode::CaptureOnly, so a long-running npm script could emit a readiness receipt but its parent would not see that receipt until the child exited. This breaks dev servers, watchers, daemons, and agent orchestration that starts a background service before launching a dependent worker.

The npm filter is line-oriented, so it can use RTK's existing streaming abstraction without guessing which script names are long-running and without disabling filtering.

Fixes #3604.

Validation

• cargo test --test npm_streaming_test
• cargo test cmds::js::npm_cmd::tests
• cargo fmt --all --check
• cargo clippy --all-targets
• cargo test — 2693 passed, 8 ignored
• manual built-binary smoke: readiness appeared in the parent process output while the npm child remained alive
• quiet-script parity: installed and patched binaries both returned ok\n, exit 0

[CLAassistant]

All committers have signed the CLA.