bug: Project leaves overlays behind for the columns it drops

Dropping a column with Operation::Project prunes the data files whose fields all went away, but leaves the fragment's overlays untouched. A fragment can therefore come out of a projection carrying a DataOverlayFile that supplies only field ids the manifest schema no longer contains.

No concurrency is involved — a plain overlay commit followed by a plain projection reproduces it.

Repro

On a dataset with columns a, b, c, d:

1. Commit a DataOverlay supplying new values for c on fragment 0.
2. Commit a Project to ["a", "b", "d"].

The resulting manifest has fragment 0 with overlays = [<file supplying field 2>], while the schema field ids are {0, 1, 3}. Field 2 is gone, so nothing will ever read that overlay, and cleanup has no reason to believe the file is dead.

Cause

Operation::Project's arm in rust/lance-table/src/transaction/manifest_build.rs:924 computes remaining_field_ids from the projected schema and retains only the data files with a surviving field:

for fragment in final_fragments.iter_mut() {
    fragment.files.retain(|file| {
        file.fields
            .iter()
            .any(|field_id| remaining_field_ids.contains(field_id))
    });
}

fragment.overlays gets the same treatment nowhere. The Merge arm above it has the same shape and the same gap.

Expected

An overlay whose field ids are all absent from the projected schema should be dropped alongside the data files, by the same remaining_field_ids rule. An overlay that still supplies at least one live field should be kept.

Found by

The legacy conflict-resolution characterization suite in #9220, which asserts that every overlay on a committed manifest supplies at least one field that is in the schema.

Start in rust/lance-table/src/transaction/manifest_build.rs around the Operation::Project arm at line 924, then compare it with the Merge arm above. Reproduce the overlay-then-project sequence described in the issue and inspect the legacy conflict-resolution characterization suite from #9220. Done means projected manifests drop overlays with no surviving field ids while retaining overlays that still supply a live field.