covers: balance-resume: cover letter and its directed test

covers: balance-resume: cover letter and its directed test

A separate, single-patch cover for "btrfs: track balance progress with
virtual address range" (kept apart from the stripe-alloc series at
Zygo's direction): the usage=90 resume heuristic it replaces, what the
cursor is and where it lives, and the directed test that holds a
paused-unmounted-resumed balance to relocating exactly the chunks below
the cursor, full ones included.  README row updated from "no cover
letter".

Assisted-by: Claude:claude-fable-5-1