Backport the recursive snapshot summary tool

[arkmish]

Description

Add an offline subtree summary to help operators locate where a saved ZooKeeper snapshot's znodes and payload bytes are concentrated, without dumping node values or scanning a live ensemble.

This adapts Apache ZooKeeper's ZOOKEEPER-4566 implementation, reusing the native snapshot loader and existing CLI conventions.

bin/zkSnapshotRecursiveSummaryToolkit.sh snapshot.1 /app 1

• Accepts a snapshot file, an existing absolute starting path, and a nonnegative display depth.
• Reports total descendant count and subtree payload bytes for non-leaf nodes. Payload bytes include the starting node's own data; descendant count excludes the starting node.
• Uses depth 0 for unlimited output. Depth limits only what is displayed: totals still cover the full subtree.
• Includes leaf and ephemeral data in the totals, while omitting individual leaf entries from the output.
• Supports uncompressed, gzip, and snappy snapshots, with explicit errors for invalid paths or corrupt input and both Unix/Windows launchers.

The tool loads the snapshot into memory and reads it without modification. Its output describes the saved snapshot, not a transaction-consistent reconstruction or current live state.

Tests

Validated SnapshotRecursiveSummaryTest, SnapshotComparerTest, and SnapStreamTest using Maven on JDK 11 with Java 8 API targeting:

Tests run: 33, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

Coverage includes root and subtree selection, leaf behavior, exact byte/count totals, display-depth boundaries, null data, ephemerals, compression, invalid arguments, corruption, and shell launcher execution.

Actual Java 8 VM and Windows launcher execution were not available.

• Local code review completed

Changes that Break Backward Compatibility (Optional)

None to existing server or client behavior. This adds a read-only command and does not alter snapshot formats or replay transaction logs.

Documentation (Optional)

Updated zookeeper-docs/src/main/resources/markdown/zookeeperTools.md with usage, sample output, depth/counting semantics, and operational limitations.

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