Improve deletion progress responsiveness and preview speed

[adxptived]

Summary

• Keep streamed delete progress visibly updating while file deletion overlaps with directory scanning, so large deletes do not appear frozen after scan progress stalls.
• Add --no-size-preview to skip expensive confirmation-preview size calculations for huge directory trees.
• Add a bulk top-level file delete path for large selections, avoiding one worker/progress item per file when deleting thousands of selected files.
• Reuse the bulk engine from the GUI for large top-level file selections while preserving symlink handling, shred behavior, cancellation, and per-file error reporting.
• Document the new preview flag and bulk-file performance behavior in the README.
• Add parser and bulk-delete test coverage plus updated CliOptions test helper construction.

Verification

• git diff --check passed in the sandbox.
• Static compile-oriented review of changed CliOptions construction sites and bulk-delete safety gates completed.
• Could not run cargo fmt, cargo clippy, or cargo test because Cargo is not installed in this sandbox (cargo: command not found).