Support reading MAP<K, BLOB> blob files #283

Motivation

Java Paimon stores MAP<K, BLOB> columns in dedicated .blob files. Paimon C++ should be able to load those schemas and read the existing payload format through the Table API, including data-evolution fallback across sequence layers.

Reader scope

• Decode payload version 1 for BOOL, integer, DATE, STRING, BINARY, and DECIMAL keys.
• Support both inline BLOB values and BLOB descriptors.
• Preserve null maps, empty maps, null values, and empty values.
• Validate payload bounds, duplicate keys, DECIMAL canonical encoding, UTF-8 STRING keys, and non-null map keys required by Arrow.
• Resolve placeholders across multiple sequence layers.

paimon.map.selected-keys is intentionally unsupported for Map Blob reads for now because filtering the internal fallback placeholder would lose older-layer data. TIME keys and ARRAY<BLOB> are also outside this issue and should be handled separately with their end-to-end type/format support.

Write support

This issue only tracks read compatibility. C++ table creation, regular writes, and append compaction must reject MAP<K, BLOB> until a Java-compatible Map Blob writer is implemented. There is no concrete writer timeline yet; writer support should be proposed and tracked separately.

Implementation: #278