Strengthen dimension label tests to verify label preservation

Address reviewer feedback: tests that claim label preservation or
propagation now explicitly verify the label string (via row_to_labels
or Shape.to_labels), not just absence of exceptions.

Key changes:
- Add get_var_label helper using Row.row_to_labels to inspect labels
- Add test 5b: verifies variable label upgrade in the environment
- Tests 1, 3: verify label visible in result shape via Shape.to_labels
- Tests 9, 11: restructure concat tests to use variable components
  (triggers s_dim_one collapse), verify label="x" and d=5
- Tests 12, 14-17: verify label="x" on solved dims after affine/stride
  arithmetic, not just absence of errors
- Test 18: assert definite Shape_error for mutual inequality with
  conflicting labels, instead of accepting either outcome

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: lukstafi

test/einsum/test_dimension_labels.expected

@@ -3,12 +3,14 @@ Found 0, in the config file
 === Dimension Label Tests ===
 
 Test 1: Same label, same size -- succeeds
-  PASS: same label unification succeeded
+  Result labels: [batch]
+  PASS: label preserved in result
 
 Test 2: Conflicting labels, same size -- raises
   PASS: got expected Shape_error: solved dimensions for axis: different labels
 
 Test 3: One labeled, one unlabeled, same size -- compatible
+  Result labels: [batch]
   PASS: labeled + unlabeled compatible
 
 Test 4: Variable-mediated: unlabeled first, labeled later -- compatible
@@ -17,6 +19,10 @@ Test 4: Variable-mediated: unlabeled first, labeled later -- compatible
 Test 5: Variable-mediated: unlabeled, labeled, then conflicting -- raises
   PASS: got expected Shape_error: solved dimensions for axis: different labels
 
+Test 5b: Variable-mediated: label upgrade verified in environment
+  Var v label after upgrade: "batch"
+  PASS: variable label upgraded to "batch"
+
 Test 6: Shape.to_labels after inference
   Labels: [batch]
   PASS: label visible in Shape.to_labels
@@ -30,39 +36,40 @@ Test 8: Operation.range ~axis_label
   PASS: range axis_label survives inference
 
 Test 9: Concat, consistent labels -- label preserved
-  Not fully solved
-  PASS: concat with consistent labels did not raise
+  Solved to d=5, label="x"
+  PASS: concat preserved label "x" with d=5
 
 Test 10: Concat, conflicting labels -- raises
   PASS: got expected Shape_error: concat: conflicting dimension labels
 
 Test 11: Concat, mix labeled/unlabeled -- label preserved
-  Not fully solved
-  PASS: concat with mixed labels did not raise
+  Solved to d=5, label="x"
+  PASS: concat preserved label "x" from labeled component
 
 Test 12: Affine, matching labels -- preserved
-  Solved to d=6
-  PASS: affine with matching labels did not raise
+  Solved to d=6, label="x"
+  PASS: affine preserved label "x" with d=6
 
 Test 13: Affine, conflicting labels -- raises
   PASS: got expected Shape_error: convolution: conflicting dimension labels between stride and kernel
 
 Test 14: Stride no-conv forward -- label propagated
-  PASS: stride forward with matching labels succeeded
+  Solved to d=8, label="x"
+  PASS: stride forward propagated label "x"
 
 Test 15: Stride no-conv reverse -- label propagated
-  Solved over to d=4
-  PASS: stride reverse with label propagation succeeded
+  Solved over to d=4, label="x"
+  PASS: stride reverse propagated label "x" to over
 
 Test 16: Conv no-padding forward -- label propagated
-  PASS: conv forward with matching labels succeeded
+  Solved to d=6, label="x"
+  PASS: conv forward propagated label "x"
 
 Test 17: Conv no-padding reverse -- label propagated
-  Solved over to d=4
-  PASS: conv reverse with label propagation succeeded
+  Solved over to d=4, label="x"
+  PASS: conv reverse propagated label "x" to over
 
-Test 18: LUB with conflicting labels -- demotion to d=1
-  INFO: Shape_error in LUB test: dimension comparison for axis: different labels
-  NOTE: LUB may raise when axes are mutually constrained
+Test 18: LUB with conflicting labels -- strict equality raises
+  PASS: conflicting labels in mutual inequality correctly raises: dimension comparison for axis: different labels
 
 === Done ===