feat: support in memory trace generation

[DavePearce]

This separates the mechanism for representing columns from trace generation itself, such that the representation can support different implementations and (critically) writing can be performed in memory.

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Note

Replaces file-mapped buffer writing with a pluggable Column interface and bitwidth-based headers, updating open/metadata APIs, generated fields, and setters to support in-memory trace generation.

• Java API and data model
  • Replace bytesPerElement with bitwidth in ColumnHeader and its construction.
  • Introduce Column interface (write(boolean|long|byte[])).
  • Change open(...) to open(Column[] columns); remove file/ByteBuffer-based open path.
  • Replace register fields from MappedByteBuffer to Column.
  • Replace addMetadata(...) with getMetaData() accessor.
• Code generation changes
  • Column setters now call Column.write(...); remove custom byte/long putters and ByteBuffer position math.
  • fillAndValidateRow() writes default values via Column.write(...) instead of advancing by byte width.
  • Remove generation of file/IO header-writing logic and related helpers; simplify utilities (drop byte-width/register-count helpers).

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