feat(table): add incremental append scan

[fallintoplace]

Summary

Adds an incremental append scan through Table.NewIncrementalAppendScan.

• Plans data files added by append snapshots between a starting snapshot and an end snapshot.
• Supports inclusive and exclusive starting boundaries plus snapshot ID and as-of selectors.
• Validates snapshot ancestry while preserving the existing file-scan task shape.

Tests

• go test ./table
• Covers default starts, snapshot ID and as-of selectors, inclusive and exclusive boundaries, expired exclusive parents, ancestry validation, and append-only snapshot selection.

Scope

This is an append-only incremental scan. Broader changelog scanning and row-level change tracking are out of scope. Branch and tag selectors can be added separately.