feat: support in memory trace generation (cherry pick)

[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.

---

Note

Replaces MappedByteBuffer-based writing with a Column interface and updates generated Java APIs (headers use bitwidth, new open signature, metadata accessor) to support in-memory trace generation.

• Java codegen (classes/interfaces):
  • Introduce Column interface with write(boolean|long|byte[]) and make open accept Column[].
  • Change register fields from MappedByteBuffer to Column; make open public and initialize Column fields.
  • Update setters to use Column.write(...); remove low-level byte/long putter logic.
  • Default-fill missing values in fillAndValidateRow() via Column.write(...).
  • Adjust headers(...) to emit ColumnHeader with bitwidth (int) instead of bytesPerElement.
  • Replace addMetadata(...) with getMetaData() accessor.
• Snippets/API:
  • Add javaColumn snippet; change javaColumnHeader to use bitwidth; simplify javaOpenSignature to public void open(Column[] columns);.
• Utils:
  • Remove now-unused helpers (byteWidth, maxValueStr, getMaxRegisterIndex) and related imports.

^{Written by Cursor Bugbot for commit b436011. This will update automatically on new commits. Configure here.}