fix: judge tombstone drops over the whole compaction range

[nepinhum]

What this changes

A compaction may drop a delete marker when no deeper level holds that key.
It checked only the range of its input files, but it also rewrites whole files pulled in from the next level which can cover much more.

So an L1 input covering b…c pulls in an L2 file covering a…z, the delete
marker for y inside it gets dropped and an older y in L3 comes back.

It now checks the range of every file it rewrites. Fewer delete markers are
dropped, never more.

How it was checked

v fmt -verify .
v vet .
v test .
v run examples/smoke

Checklist

• v fmt -w . ran
• Tests pass
• A new module carries a README.md

[nepinhum]

@xRookieFight review 🤣

[xRookieFight]

Approved