Add concurrent per-column filling API

[k-barseghyan]

Summary

Adds a generated synchronous per-column filling API that allows callers to populate distinct columns concurrently.

The existing row-oriented and batch APIs remain available and unchanged.

API

TradeStore store = TradeStore.create(expectedSize);
TradeStore.ColumnAppender columns = store.columnAppender();

columns.timestamp(timestampValues);
columns.price(priceValues, fromIndex, toIndex);
columns.symbol(symbolValues);

store.seal();

Semantics

• Each call copies its values synchronously and does not retain the source array.
• Different columns may be filled concurrently.
• Calls targeting the same column must be externally serialized.
• Each column maintains its own count and capacity.
• Row/batch mutation and per-column mutation cannot be mixed on the same store.
• The store creates no executors, futures, latches, phasers, or cyclic barriers.
• The caller owns task execution and joins all column tasks before calling seal().
• seal() never waits. It validates that all column counts are equal and remains retryable when validation fails.
• columnAppender() returns the same store-owned appender instance.

Compatibility

• Preserves the published 1.2.0 generated constructors, factories, row append, batch append, cursor, view, and sealing APIs.
• Adds columnAppender() as a Java 8 default method so previously compiled external implementations continue to link.
• External implementations that do not support the new capability receive UnsupportedOperationException.
• Handles nested generated-name collisions, including legal top-level types containing $, while keeping incremental generation stable.
• Documents and tests the source-compatibility boundary for implementations that already declare an incompatible columnAppender() method.

Verification

• ./mvnw clean verify: 146 tests passed.
• Strict project and generated-source Javadocs passed.
• Generated stores and consumers compile with javac --release 8.
• Generated class files use Java 8 bytecode, major version 52.
• Compatibility was checked against the published 1.2.0 artifacts and precompiled 1.2 consumers and implementations.
• Collision, incremental-generation, named-module, artifact-boundary, and 1,500-column schema tests passed.
• Generated sources contain no executor, future, completion-stage, latch, phaser, or barrier dependencies.
• Native JDK 8 execution remains covered by the Java 8 CI job.

Closes #2