`MaskSource` could say "instance 3 of that segmentation run" but had no way
to say "the sky". Adding `Category { signature, name }` beside `Subject` is
what lets a local adjustment attach to a semantic category at all.
Stored as identity like a subject, and for the same reason: the coverage is
megabytes and is reproducible by running the same model over the same image,
so the sidecar carries what finds it again and the session carries pixels.
## A name rather than an index
An index would be smaller and would match `Subject`. It would also be a bug.
The grouping lives in `models/scene/categories.txt`, which is editable by
design — adding one category to it renumbers every category after it, and
every stored layer would silently start grading something else. A name that
no longer exists is simply not found and the layer reads as stale, which is
the failure that announces itself.
Staleness is otherwise identical to a subject's: the coverage buffer is in
the session, never the sidecar, so a signature from another run points at
pixels that were never computed.
## Two tests, and the second one caught a real shape
Round-tripping the name matters more than usual here, because the whole
argument for storing a name instead of an index is worthless if the sidecar
is what drops it.
The multi-word case is the one worth having: `category = swimming pool` is
written on one line, and a reader splitting on whitespace would have
truncated it to a category no model has — a layer that silently masks
nothing. `category` is also its own key rather than a reuse of `class`,
because a file conflating them would round-trip a subject into a category.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
dtourolleandClaude Opus 5
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