CheckpointStore 0.6.0 API

All Implemented Interfaces:
```
public final class CheckpointStore

                    
```
Resumable transfer checkpoints with conditional writes (HEL-236 scenario 4).
Design: checkpoints are an APPEND-ONLY sequence of immutable objects (<prefix>/checkpoint-<seq>.json), each created with WriteCondition.IfAbsent. Advancing means creating sequence latest + 1; when two workers race, exactly one create succeeds and the loser gets a PreconditionFailedException — it must reload latest and re-plan, so concurrent workers can never silently overwrite each other's progress. Deliberately built on CONDITIONAL_CREATE only (universally the best-supported conditional primitive across S3-compatible providers) rather than etag-If-Match update-in-place.
The payload is caller-defined (Checkpoint.data); keep it a POSITION (keys, offsets, watermarks) — never row values or credentials.

Nested Class Summary

Nested Classes
Modifier and Type	Class	Description
`public final class`	`CheckpointStore.Checkpoint`

Field Summary

Fields
Modifier and Type	Field	Description

Constructor Summary

Constructors
Constructor	Description
`CheckpointStore(ObjectStore store, String prefix, Clock clock)`

Enum Constant Summary

Enum Constants
Enum Constant	Description

Method Summary

Modifier and Type	Method	Description
`final CheckpointStore.Checkpoint`	`latest()`	The newest committed checkpoint, or null when none exists yet.
`final CheckpointStore.Checkpoint`	`save(String data, Long expectedSequence)`	Commit the next checkpoint.

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

- Constructor Detail
  - CheckpointStore
```
CheckpointStore(ObjectStore store, String prefix, Clock clock)
```
- Method Detail
  - latest
```
 final CheckpointStore.Checkpoint latest()
```
    The newest committed checkpoint, or null when none exists yet.
  - save
```
 final CheckpointStore.Checkpoint save(String data, Long expectedSequence)
```
    Commit the next checkpoint. expectedSequence is the fence: pass the sequence you resumed from (or null for "first checkpoint") and the write lands at expectedSequence + 1 — if another worker got there first this throws PreconditionFailedException WITHOUT touching its progress.