feat: pointer-free topology snapshots

[pathscale]

Was a local-only branch that had never been pushed, recovered during a sweep
for uncommitted work. Opening it so it survives; this is a recovery PR, not a
claim that it is ready.

One commit (275aacd), 152 lines across src/concurrent/map.rs and
src/concurrent/set.rs.

Needs a decision: WorkTablesIndex#14 is in the beta.17 stack and touches the
same concurrent structures. Someone who knows this work should say whether it is
superseded by #14, complementary to it, or wanted afterwards. If it is dead,
close it deliberately rather than leaving it to be rediscovered by the next
sweep.

[pathscale]

Closing: this was not lost work. fix/range-batch already carries the same commit (2fca45f, identical title), so #14 contains it and the orphan branch was a duplicate. My earlier claim that it was unpushed treasure was wrong.

The branch stays pushed for reference. One PR per repo: #14.