New invariant for many to many conumer producer lanes

.agents/invariants/GRAPH_COMPUTE_BATCH_INVARIANT.md

@@ -225,6 +225,26 @@ compile-time shaping needed for a scheduled consumer must be encoded inside the
 corresponding `spat.deferred_communication` body. Phase 2 must not recover
 missing graph semantics by inspecting consumers after the deferred operation.
 
+### Consumer and producer lanes are many-to-many
+
+A graph consumer lane may read fragments from several producer lanes. Several
+consumer lanes may also read the same producer lane. Therefore:
+
+- consumer and producer batches need not have the same lane count;
+- consumer lane `i` must not be assumed to depend on producer lane `i`;
+- scheduling dependencies must follow the statically analyzable physical-slot
+  projections expressed by the consumer body;
+- an unproven projection must not silently drop producer dependencies or
+  reclassify a produced graph value as a host input.
+
+For example, one output-row lane of a `3x3` convolution normally reads up to
+three input-row lanes. Output-channel tiling adds another projection dimension:
+several output-channel-tile lanes may read the same set of input-row lanes.
+
+When exact projected dependencies cannot be proven, scheduling may conservatively
+depend on the complete producer batch, but communication realization must still
+preserve the consumer body's actual selection semantics.
+
 ## Graph lane, scheduled lane, and physical core are different identities
 
 These concepts must never be conflated: