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/module-internals.md

@@ -492,7 +492,7 @@ stage.
 router.
 
 [`invisible_engine.py`](../src/remove_ai_watermarks/invisible_engine.py) handles
-image sizing, optional pre-upscaling, postprocessing, and the public engine
+image sizing, postprocessing, and the public engine
 interface. It delegates model execution to
 [`_internal/watermark_remover.py`](../src/remove_ai_watermarks/_internal/watermark_remover.py).
 
@@ -725,18 +725,20 @@ Regression coverage:
 
 - [`test_tiling.py`](../tests/test_tiling.py)
 
-### Upscaling and postprocessing
-
-[`upscaler.py`](../src/remove_ai_watermarks/upscaler.py) is the optional
-Real-ESRGAN path used only when enlarging a small image to the minimum
-resolution floor. Failure or an absent extra falls back to Lanczos.
+### Postprocessing
 
 [`humanizer.py`](../src/remove_ai_watermarks/humanizer.py) contains explicit
 grain, unsharp masking, and adaptive polish helpers.
 
+`upscaler.py` held an optional Real-ESRGAN path, reachable only when enlarging a
+small image to the minimum-resolution floor. That floor existed to lift small
+inputs toward SDXL's ~1024 training size; when the SDXL profiles were removed it
+was forced to 0 on every path, so the module, the `--min-resolution` and
+`--upscaler` options and the `esrgan` extra were all unreachable and went with
+it. Only the `max_resolution` cap can move geometry now, and it only scales down.
+
 Regression coverage:
 
-- [`test_upscaler.py`](../tests/test_upscaler.py)
 - [`test_humanizer.py`](../tests/test_humanizer.py)
 
 ## Image input and output