fix(gc): a secondary index must not be silently collectable

[whilo]

The gap

datahike.gc unions mark-from-key-map over every stored secondary key-map and treats the result as the reachable set (gc.cljc:99-103). Whatever that returns is the only thing standing between an index and the sweep.

Nothing tested it — at any level, for any backend. Not stratum's ~40-line commit-chain walk, not scriptum's or proximum's #{}, not the union in gc.cljc. Grepping mark-from-key-map across test/ returns nothing.

This adds the first coverage: a real file-backed collection that must leave a scriptum index searchable afterwards and the primary data intact.

Why it matters now

#{} is the honest answer for an index whose bytes live on the filesystem — konserve holds nothing of it to mark. It becomes "delete the whole index" for one whose bytes live in konserve. Same value, opposite meanings, and no error in either direction.

That transition is imminent (replikativ/scriptum#3 moves scriptum's segments into konserve), and the failure mode is silent and total.

So a key-map now declares where its storage lives:

(sec/konserve-backed? key-map)   ; => (= :konserve (:backing key-map))

and the default method refuses rather than guesses — an index declaring :konserve with no marking implementation raises instead of reporting itself unreachable. A broken GC run is a cheap failure; a silently deleted index is not.

scriptum's method keys off the same declaration, so wiring it to scriptum.konserve cannot land without the marking landing too.

Notes

• GC tests must be file-backed. :memory refuses to mark at all ("Index needs to be properly flushed before marking"), so a GC test on it proves nothing about GC. Cost me one debug cycle; worth knowing.
• Backwards compatible: a key-map without :backing behaves exactly as before.
• Full secondary suite green: 87 tests, 396 assertions.

Not in this PR

The actual konserve marking for scriptum — returning the branch manifest plus every blob it references — lands with the wiring, once scriptum exposes a writer over its konserve Directory. The guard here is what makes that ordering enforceable instead of merely intended.