feat: allow KVS initialization from an explicit snapshot

[atarekra]

Closes #237

The problem

The KVS keeps older snapshots so that a damaged current file isn't fatal, but
Kvs::open() only ever read snapshot 0 — the path was built with a hardcoded
"_0". If that file was missing or corrupt, the KVS refused to initialize even
when _1, _2 and _3 were present and intact. The recovery data existed and
nothing in the API could reach it.

The change

Kvs::open() takes an optional snapshot ID, defaulting to SnapshotId(0), and
KvsBuilder gets a matching snapshot_id() option:

auto kvs = KvsBuilder(InstanceId{1}).snapshot_id(SnapshotId(1)).build();

The default keeps every existing caller working unchanged.

I went with the first option from the issue (explicit builder parameter) rather
than the automatic retry loop, for the reason given in the issue itself: an
implicit fallback means the application starts up on data of unknown age with no
signal that anything went wrong. So a missing snapshot still fails, and
need_kvs_flag() decides whether that's an error or an empty KVS. There's a test
that pins this down deliberately, so the fallback doesn't get added later by
accident.

Tests

7 new tests: the default path still reads snapshot 0; opening from an explicit
snapshot works with _0 deleted; a missing snapshot fails rather than falling
back; Optional semantics apply to the requested ID; and the full recovery story
(open an older generation, flush, confirm it becomes the current KVS).

Notes

• Rust has the same limitation — kvs_builder.rs also hardcodes SnapshotId(0).

[github-actions]

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INFO: Analyzed target //:license-check (177 packages loaded, 11510 targets configured).
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INFO: Build completed successfully, 17 total actions
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[github-actions]

The created documentation from the pull request is available at: docu-html

[Ahmed-Elsaka-JC]

snapshotId already initialized on builder constructor , why do you reintialize it here again ? most probably to avoid updating testing.