[Feature] Roadmap for Paimon C++ 0.4.0

Inizia dalle issue mirate collegate, in particolare #158 per le scritture in tempo reale e le letture union, #174 per la produzione del changelog, #170 per Format Table e #169 o #204 per Data Evolution. Leggi i documenti PIP di riferimento quando applicabile e scegli un elemento della roadmap con un ambito ben definito. Per considerarlo completato è necessaria una modifica mirata e verificabile, con i relativi test unitari e di integrazione, la documentazione e la validazione della compatibilità.

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Motivation

Looking ahead, Paimon C++ will focus on several areas:

• Storage and retrieval optimizations for vertical workloads, including efficient MAP storage, richer data types such as VECTOR, and complete index support.
• Making newly written data queryable in real time through pluggable real-time writes and memory/disk union reads.
• Following up on important capabilities from the broader Paimon community, including changelog production, Format Table, and extensions to Data Evolution.

Solution

1. Storage and retrieval optimizations for vertical workloads

• Support shared-shredding columnar storage for MAP<STRING, T> based on PIP-43, including adaptive physical columns, field mappings, overflow storage, and end-to-end reads and writes.
• Support richer data types, starting with VECTOR<T, N> based on PIP-40, including schema representation, storage, reads, writes, and Data Evolution.
• Complete File Index support by adding index generation to the existing read path.
• Continue improving storage layout, predicate pushdown, column pruning, point lookups, and index-assisted retrieval for workload-specific scenarios.

2. Making real-time data queryable

• Support pluggable real-time writes and memory/disk union reads based on PIP-46, tracked by #158.
• Make data held by an active writer queryable before it is committed into a snapshot.
• Guarantee consistent query results without missing or duplicate rows while writing, committing, and reclaiming memory segments concurrently.
• Support append tables first, followed by primary-key tables, deletion-vector mode, and additional production scenarios.

3. Following up on Paimon community capabilities

• Support changelog production, including input, lookup, and full-compaction modes, tracked by #174.
• Support Format Table reads and writes for Hive-style file directories, tracked by #170.
• Extend Data Evolution to work with deletion vectors, tracked by #169.
• Extend Data Evolution to primary-key tables and support compaction across evolved field groups, tracked by #204.
• Continue following relevant Paimon features and make them available through native Paimon C++ APIs where appropriate.

Each roadmap item should be delivered through focused issues and reviewable pull requests, with unit tests, integration tests, documentation, and compatibility validation where applicable.

Anything else?

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