Add deterministic divisor oracle for #54

[isomorphisms]

Refs #54.

This deliberately starts from main, not experiment/powervr-phone-fastpath.

Oracle

Adds a host-side mathematical oracle under _/oracle/ and runs it in the normal test workflow. The acceptance sequence is:

1. one stationary pole plus one exactly coincident zero reduces algebraically to the empty divisor, including at the removable point;
2. a slightly displaced pair agrees with the direct complex ratio and is not simplified away;
3. 24 exact pairs reduce to the empty divisor, while 24 displaced pairs agree with a direct ratio product;
4. moving wandering poles and physical-GPU optimization remain later acceptance boundaries.

Failures isolated

The tests keep two recent evaluator strategies only as negative controls, not as architecture:

• divisor-first fp32 accumulation followed by a large q can erase a real displaced-pair contribution;
• the raw collapsed fp32 product of 24 pole factors can overflow before phase/log-modulus extraction.

There is also a more basic semantic boundary: exact cancellation has to happen before numeric evaluation. Leaving the pair in separate atan/log paths makes the removable point reach atan(0,0), whose GLSL result is undefined.

The oracle therefore does not preserve the recent raw/scaled product helpers, marker-culling work, or their structural green tests as constraints. It defines the mathematical behavior a replacement live evaluator must match.

Verification

python -m unittest discover -s _/oracle -p 'test_*.py' -v passes 7/7 locally.

This PR is intentionally draft: it establishes the deterministic oracle and failure ordering first. It does not claim live GLSL parity or physical PowerVR acceptance yet.