fix(PROC-1927): respect compress flag in _handle_new

[CalNightingale]

The compress flag is plumbed from runner.py (--compression-mode=PRESERVE → compress=False) through ingest_instances → _handle_new, but was then dropped: _handle_new called _compress_instances() unconditionally, making --compression-mode=PRESERVE a no-op. Any caller wanting to skip JPEG2000 encoding (e.g. DICOMs whose pixel data won't encode cleanly) hit compression errors regardless.

• Gate the _compress_instances() call in _handle_new on the compress flag; when False, instances pass through validated-but-untranscoded with no compression errors.
• Add test_append_no_compress asserting compress=False preserves the original transfer syntax and file size.

Surfaced while seeding the gh-laplace-combo test environment, where 16×16 noise frames openjpeg can't encode and PRESERVE failed to escape compression. A local patch of this shape on Cal's laptop should be reverted once this lands.

🤖 Generated with Claude Code

[Alceu-wv]

LGTM