datalake_fdw: read path — planned fragments, CustomScan, projection and pruning #2019

Part of #2008. Letters (A, B0–B7, C, D, E) are the PRs listed there; this is D.

Scope

• Fragments from the engine's plan (files, row-group ranges) assigned to segments; one large file may be split by row group.
• A CustomScan (planner hook, design decision D10) that opens a reader per fragment and decodes batches into slots.
• Projection by field id from the table's Iceberg schema (ProjectionSet), so files written before a column was added or renamed read correctly; only needed columns are read.
• Row-group pruning from the query's quals against Parquet statistics; EXPLAIN ANALYZE reports row groups skipped.
• With it: datalake_fdw: Iceberg name mapping for data files without field ids #1989 name mapping for files without field ids.

Out of scope

Delete files (E), time travel (#1683 §2.3), ANALYZE beyond the current zero-sample no-op.

Depends on

A, B2, B4. Skeleton, projection and pruning are testable against the stub and local files first.

Acceptance

• Same rows as Spark on the same table, including one Spark evolved (column added, renamed, dropped, int promoted to long).
• A file split into three fragments reads the same as whole; every segment reads only its fragments.
• EXPLAIN ANALYZE shows pruning for a selective predicate and none for WHERE true.
• Unsupported or dictionary-encoded columns are refused naming the column; cancel mid-scan releases the reader.