Per-source confidence grade + source identity on tiles (CATZOC-style)

[bkeepers]

The mariner must be able to tell GEBCO-interpolated deep ocean from a surveyed 3 m CUDEM coastline.

Carry a per-source quality grade (analogous to ENC CATZOC zones of confidence) and source id from each source's metadata.json through ingest into the tiles.

MVP: a source-id + coarse confidence attribute on tiles and a viewer affordance to inspect it.

Related: the source-footprint provenance layer (#19) is the cheapest first step; GEBCO TID masking (#18) feeds a per-pixel provenance band later.