Undefer AI subject masking, which shipped, and give it a clause

§7 still listed "AI subject masking — deferred per D11" while
MaskSource::Subject and MaskSource::Category, backed by dr-segment's
instance and semantic models, had been the primary way a local
adjustment is made for weeks. The code was tagged FR-DEV-3, which
names gradients and brushes and says nothing about a model.

FR-DEV-3i now states what exists: a subject or a category found by a
local model, stored as identity with the run's signature so that it
merges per field and reads as stale rather than wrong, then treated as
any other layer by the edge, stroke, composition and reveal clauses.
The one place it departs from FR-DEV-19 — coverage written run-length
coded beside the layer, so a stored subject renders without a model —
is recorded in the clause instead of left for the next audit to find.
The segmentation crate and the UI's selection module are tagged to it.

core/dr-pipeline/src/mask.rs

@@ -555,6 +555,7 @@ impl Morphology {
     ];
 }
 
+/// TRACES: FR-DEV-3 | FR-DEV-3i
 /// Where a mask layer applies.
 #[derive(Debug, Clone, PartialEq)]
 pub enum MaskSource {