- Merge `origin/main` (up to #21546) into the `performance` branch.
- Conflicts resolved by taking main's finalized form where the perf
branch was behind (`ExistenceFilterVersion`→1 per #21164,
`mdbx-go`→v0.40.1, `merge.go` `findMergeRangeInFiles` refactor, dropped
`erigon-snapshot` module dep, fusefilter deferred-close refactor,
`Versions.MustSupport`, atomic per-key prune throttle).
- Adopted main's collation-at-tip design (`CollateAndPrune` in the FCU
path, #21398/#21415) and removed the perf branch's older
`frozenBlocks`-gating (`SetFrozenBlocksProvider`/`MaxCollatableTxNum`,
`db/services/snapshot_progress.go`, its gating tests, and callers).
- Verified: `make erigon integration` build, `make lint` (clean), `make
test-short` (green).

## Summary

Re-enable the sharded FuseFilter existence-filter format by setting
`ExistenceFilterVersion` back from `1` to `2` in
`db/recsplit/recsplit.go`. This is the switch that erigontech#21164 turned off
after erigontech#20644 introduced the format. New index files (`.kvi`/`.efi`) will
again embed the sharded BinaryFuse filter (256 shards by `keyHash>>56`),
which cuts existence-filter build-time RAM ~256× (the motivation was
bloatnet 3B-key `.efi` builds).

## Why it's safe

- **Read path is version-aware.** `db/recsplit/index.go` reads the
embedded `dataStructureVersion` per file and dispatches to v0/v1/v2
readers, so existing v1 files stay readable and new v2 files are read by
the matching `case 2` path.
- **Accessor/existence files are built locally, not seeded.**
`SeedableV3Extensions()` publishes only `.kv/.v/.ef/.ap`;
`.kvi/.efi/.kvei` are rebuilt locally via `BuildMissedAccessors`, so
this does not affect the shared snapshot set.

## Test plan
- [ ] CI green

Co-authored-by: JkLondon <me@ilyamikheev.com>