fix metadata loss in to_geff

[TeunHuijben]

Fix graph metadata loss in to_geff + add graph-less metadata reader

Problem

Downstream libraries need shape (the dense segmentation shape) to survive a geff
round-trip. Two things blocked that:

1. to_geff silently dropped all graph metadata when the caller supplied its own
   GeffMetadata — the merge into extra["tracksdata"] only ran in the
   geff_metadata is None branch. from_geff already read the extras back, so only
   the write path was broken.
2. No way to read that metadata before you have a graph, so callers hardcoded
   extra["tracksdata"]["shape"] — a private detail we couldn't move.

Changes

• to_geff merges graph metadata into extra["tracksdata"] for caller-supplied
  metadata too. Caller entries win on collisions; the caller's object isn't mutated.
• New tracksdata.io.read_graph_metadata(source), taking a store/path or an
  already-parsed GeffMetadata. Returns {} for non-tracksdata geffs.
• Docstring notes shape is the canonical key (matching GraphArrayView/to_ctc).

Note: values pass through JSON, so (5, 100, 100) reads back as [5, 100, 100].

[JoOkuma]

LGTM.
An optional suggestion, it might be worth considering merging io._geff_metadata and io._geff_dtypes

[TeunHuijben]

Thanks! I merged the files

[JoOkuma]

@TeunHuijben awesome, feel free to merge whenever you're ready