Track encoded file concatenation and column slices

Goal

Land a single low-level primitive for concatenating compatible encoded Lance files and FileFragment APIs for independently writing and combining complete physical-row slices of existing columns.

Context

• Proposal: https://github.com/lance-format/lance/discussions/8615
• Preparatory API rename: https://github.com/lance-format/lance/pull/8622
• Rust implementation: https://github.com/lance-format/lance/pull/8660

Contract

• Compatible current-version files concatenate encoded page payloads without decode and re-encode.
• Unsupported layouts use the existing ordered decode/re-encode fallback; corrupt or missing inputs remain errors.
• Column slices are immutable, fragment-local physical-row intervals with exact field and source-snapshot identity.
• Publication requires complete, gap-free, non-overlapping fragment coverage.
• Compaction reuses the same encoded-file primitive without adopting the column-slice lifecycle.
• Python ColumnSlice values support a versioned bytes round trip and pickling.

Out of scope

• Task planning, scheduling, retries, progress, recovery, or distributed commit coordination.
• A new manifest layout or transaction operation.
• Caller-owned staged-file retention and cleanup.

Completion gates

• https://github.com/lance-format/lance/pull/8660 is merged.
• Python serialization and fragment bindings implement the proposal contract.
• Encoded concatenation, fallback, malformed-input, coverage, and compaction-reuse tests pass.
• Public API and ownership boundaries are documented.