options: add Unsafe.AllowMissingWALDir option

This unsafe option allows opening a store after moving or losing the
previous WAL dir. This is understood to be risky and have associated
data loss if any of the necessary WALs are not in the new directory.

Fixes #5421

pebble-release-2.1: change path to v2

manifest: change panic to Fatalf

This panic causes an "unlock of unlocked mutex" panic on unwinding,
which obscures the original panic. Change to a `Fatalf`.

Fixes #5225

go.mod: update github.com/cockroachdb/swiss to support Go 1.25

The Go 1.25 incompatibility found in
* Homebrew/homebrew-core#226636

then:
* reported upstream in cockroachdb/swiss#48
* fixed upstream cockroachdb/swiss#49

db: fix obsolete file metric underflows

Previously a race existed in the calculation of obsolete file metrics resulting
in underflow. The values within versionSet.metrics were reset and recalculated
to reflect the set of files in versionSet.obsoleteTables whenever new files
were added to obsoleteFiles. Additionally, the cleanup manager invoked a
callback to decrease versionSet.metrics whenever a table was deleted.

The recalculation of versionSet.metrics could reset the metrics to less than
the sum of outstanding pending deletes. When the cleanup manager eventually
deleted the pending tables, these metrics would underflow.

This commit fixes the bug by maintaining separate stats for all files that have
been enqueued for the cleanup manager and all files that have been successfully
deleted. The volume of outstanding, pending deletions is the difference between
the two.

For now, there's an additional wart that the set of files that are sitting in
versionSet.obsoleteTables are still separately tracked.

Informs #4811.

go.mod: update deps to support go1.24

go.mod: update github.com/prometheus/client_golang to v1.15.0

sstable: update testutil readall to return an error

ReadAll is importing testutils which is pulling testify and other
test dependencies in the final binary

update function to return an error without calling testutils

pebble-release-2.0: change path to v2

A go module with version `v2.0` must have a path that ends in `/v2`;
otherwise `go get` gives you a "module path must match major version"
error.

This commit enacts this change for the `pebble-release-2.0` branch. In
the future we will consider making this change on master as well so we
don't have inconsistency between the branches.

Fixes #4271

pebble-release-1.1: db: fix Open issue with single-item relative paths

Previously Open with a relative path containing a single element would fail on
real filesystems, because Open would attempt to open the empty path.

Fix #3842.