Apply Select Params to spool inputs and avoid default source loading

[d-chambers]

Summary

• Apply incoming SelectParams to spool inputs before metadata and annotation filtering.
• Keep input-backed Spool widgets idle when no source is configured instead of auto-loading a default example.
• Allow external derzug.widgets providers while keeping core DerZug registry contract tests scoped to DerZug implementations.

Validation

• prek run --all-files
• env MPLCONFIGDIR=/tmp/mpl pytest tests

Result: 1552 passed, 46 skipped.