With bf-tree 0.5.3+ taking the CPR snapshot destination as a cpr_snapshot(path)
argument, the OnFlush / checkpoint / migration paths can snapshot straight to
their final per-flush / per-checkpoint / migration file instead of snapshotting
to a shared scratch file and File.Copy-ing it.

The scratch+copy was a workaround for 0.5.0, where the snapshot path was fixed in
Config and bftree held a persistent file descriptor for it (so a finalized file
had to be a copy, and racing consumers shared one snapshot via WaitForSnapshot).
In 0.5.4 cpr_snapshot creates a fresh, self-contained file per call, so:
- each site snapshots directly to its destination (no scratch file, no copy);
- the per-tree SnapshotInProgress claim is kept (bftree silently no-ops a racing
  cpr_snapshot), so concurrent flush/checkpoint/migration serialize and each
  produces its own destination file;
- TreeEntry.WaitForSnapshot is removed (no longer needed).

The scratch path remains only as the use_snapshot enabler passed at tree
create/recover time; the scratch file itself is no longer written.

Verified: RangeIndex suite (63) and cluster migrate RangeIndex suite (19) pass.

Co-authored-by: Copilot <[email protected]>

Update the RangeIndex dev doc to match the bf-tree 0.5.4 snapshot flow:
CprSnapshotByPtr now takes the destination path and the OnFlush / checkpoint /
migration paths write the CPR snapshot directly to the per-flush / per-token /
migration file (no scratch.cpr staging + File.Copy). scratch.cpr remains only as
the use_snapshot enabler passed at create/recover; it is no longer written.

Co-authored-by: Copilot <[email protected]>

…shot docs

- Add TreeEntry.SnapshotUnderClaim(destinationPath): single home for the
  spin-claim + cpr_snapshot + release pattern previously duplicated across
  OnFlush (live/cold), checkpoint, and migration paths. Behavior unchanged
  (still spins on the per-tree atomic).
- Refresh stale doc comments left by the snapshot-direct refactor: drop the
  obsolete 'File.Move/copy the scratch file' wording in the checkpoint summary
  and in the bftree_cpr_snapshot FFI docs (C# + Rust).
- Document the CprSnapshotByPtr caller contract: it does not self-serialize;
  a racing cpr_snapshot on the same tree silently no-ops while still returning
  success, so callers must hold external per-tree serialization.

Co-authored-by: Copilot <[email protected]>

The BfTreeInterop test project was orphaned: not referenced by Garnet.slnx
(so CI never built it) and still calling a long-removed API (Snapshot(),
RecoverFromSnapshot), leaving it uncompilable.

- Rewrite the snapshot/recovery tests against the current API: snapshot via
  the static CprSnapshotByPtr(handle, path) and recover via
  RecoverFromCprSnapshot(path, enableSnapshots, backend). Round-trips now
  snapshot to a distinct file and recover from it, mirroring real RangeIndex
  usage.
- Replace tests asserting outdated behavior: nonexistent-file recovery now
  throws (native returns null) instead of 'creates empty'; memory-backed
  snapshot is now a working round-trip (bftree supports CPR for memory-backed
  trees) instead of the old 'throws NotSupportedException' premise.
- Add argument-validation tests for null handle / empty paths.
- Fix latent analyzer/format issues surfaced now that the project builds
  (IDE0300, final-newline).
- Add the project to Garnet.slnx so CI builds it on every TF/OS/config and
  catches API drift like this in the future.

All 38 tests pass on net8.0 and net10.0.

Co-authored-by: Copilot <[email protected]>

Move test/BfTreeInterop.test -> test/standalone/BfTreeInterop.test so it lives
with the other standalone test projects and is reachable by the matrix command
'dotnet test test/standalone/<name>'. Add 'BfTreeInterop.test' to the
test-garnet-standalone matrix so CI actually runs it (not just builds it), and
update the relative ProjectReference depth and the Garnet.slnx path for the new
location.

Co-authored-by: Copilot <[email protected]>

The BfTreeInterop.test project was already orphaned and non-compiling before
this branch (not in Garnet.slnx, calling a long-removed API). Reverting the
revival keeps this PR focused on the snapshot refactor:

- Restore the project to its original test/BfTreeInterop.test/ location and
  prior contents.
- Remove it from Garnet.slnx and from the ci.yml standalone test matrix.
- Drop the BfTreeInterop.csproj output-root native-copy fallback that was only
  needed to run this test in CI.

Co-authored-by: Copilot <[email protected]>