chore: slim PR to user-facing essentials

Carry only the code, tests, and user-facing docs that matter to end
users; drop the modernization scaffolding accumulated while building it.

- Remove the bench harness, results, and environment captures (bench/).
- Remove internal docs and research spikes (docs/).
- Remove the Makefile; tests run via uv / pytest directly.
- Strip the bench harness and quantization-matrix steps from the Tier 2
  workflow. The golden-image test drives conversion through the Core ML
  Converter node at runtime, so no separate convert step is needed.
- Replace phase/handoff annotations across code, tests, and config with
  neutral docstrings and comments.

tests/unit/test_characterization_inputs.py

@@ -1,10 +1,10 @@
-"""Phase 2 characterization tests for coreml_suite.models.CoreMLInputs.
+"""Characterization tests for coreml_suite.models.CoreMLInputs.
 
 Locks the shape transforms applied by chunks() and coreml_kwargs() for the
 four model variants the suite supports: SD1.5, LCM (SD1.5 + timestep_cond),
 SDXL base (time_ids len 6), and SDXL refiner (time_ids len 5).
 
-These contracts feed the Core ML UNet at runtime; if Phase 3 silently
+These contracts feed the Core ML UNet at runtime; if a refactor silently
 re-shapes them, generation breaks.
 """
 import numpy as np