Summary

Make Tensor.broad respect tags when both input and output shapes are fully tagged, and make Shape.canBroadcastTo reject fully tagged positional matches that do not map by tag.

Issues

Fix #68

Why

The issue case .{ .a = 8, .l = 1, .k = 16 }.broad(.{ .a = 8, .k = 16, .v = 16 }) was accepted by positional broadcasting even though the tags do not describe the same axes. Fully tagged shapes should match and broadcast by tag, not by position.

The implementation keeps the integer scalar broadcast compatibility hook ahead of tagged broadcasting and excludes rank-0 shapes from the tagged branch so backend compatibility behavior is preserved.

Test verification (RED -> GREEN)

Command: ./bazel.sh test //zml:test

RED with the production fix reverted while keeping the new tests:

53/142 shape.Shape.test.canBroadcastTo rejects fully tagged positional mismatch...FAIL (TestUnexpectedResult)
114/142 tensor.Tensor.test.Tensor.broad maps fully tagged axes by tag...thread ... panic: Can't broadcast Tensor({a=1,k=16,f32}) to {k=16,a=8,f32}
//zml:test                                                               FAILED in 23.5s
Executed 1 out of 1 test: 1 fails locally.

GREEN on the final branch:

INFO: Build completed successfully, 6 total actions
//zml:test                                                               PASSED in 7.5s
Executed 1 out of 1 test: 1 test passes.