Core: Add V4 scan task planner

[anoopj]

Adds the planner that walks a v4 adaptive metadata tree and emits FileScanTasks. It reads the root manifest, emits a task per live DATA entry, and expands DATA_MANIFEST entries into their leaf manifests. Leaves are read lazily and, when a worker pool is supplied, expanded in parallel.

Keeping it package-private and not wired into any scan yet.

Support for the following features will be added in followup PRs.

• Inherited tracking: sequence numbers, snapshot id, and first-row-id aren't applied
• Manifest-level pruning: e.g. using the manifest_info count based short-circuit, stats/partition pruning
• Delete content, manifest deletion vectors, encryption, and non-v4 leaves
• Content cache + eager-fetch of manifests