Remove the unreachable ESRGAN upscale chain

The min-resolution floor lifted small inputs toward SDXL's ~1024 training size,
and Real-ESRGAN was an optional way to do that lifting. Both surviving profiles
run at native geometry, so the engine forced the floor to 0 on every path; the
floor never fired, `upscaling` was never true, and nothing downstream of it could
execute. Gone: upscaler.py, _esrgan_upscale, the min_resolution and upscaler
parameters, _target_size's floor branch, --min-resolution, --upscaler,
_warn_if_esrgan_unavailable and the `esrgan` extra. max_resolution stays and is
now the only lever on geometry; it can only scale down.

scripts/smoke_matrix.py was the one live consumer and neither gate saw it -
Pyright is scoped to src/ and Ruff cannot resolve its function-local import - so
`--diffusion` would have died at import. Its knob rows were written for the
removed profiles besides (--pipeline sdxl, --steps 20, --guidance-scale 5.0,
--device mps), so they are rewritten rather than patched: most now assert a knob
is REJECTED, which is the coverage worth having when the CLI accepts a value the
library refuses several layers down. Accepted-knob rows skip without CUDA, so the
row count is host-dependent and verification-plan.md no longer claims a fixed 68.

This removes a public module, a CLI option and a published extra, so the next
release is 0.25.0, not a patch.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/python-api.md

@@ -422,7 +422,7 @@ engine = InvisibleEngine(pipeline="sdxl-zimage")
 The `qwen-zimage` extra must be installed for that profile.
 
 The full `remove_watermark` signature includes strength, steps, guidance,
-seeding, tiling, resolution, upscaling, and postprocessing controls. Read the
+seeding, tiling, resolution, and postprocessing controls. Read the
 method signature in
 [`invisible_engine.py`](../src/remove_ai_watermarks/invisible_engine.py) or use
 the CLI guide for the concepts.