Align default --steps/--cfg with recommended RAW settings (52 / 3.5)

[asciidiego]

What: default --steps 28→52 and --cfg 4.5→3.5 in inference.py, sampling.py, and the README Options table.

Why (grounded in this repo + internal decision):

• The default checkpoint is oss_raw (inference.py --checkpoint default="oss_raw"), and this README's own RAW quickstart prescribes --steps 52 --cfg 3.5. The defaults (28 / 4.5) contradict our recommended RAW invocation, so a no-flag run produces a worse RAW result.
• 52 steps / guidance 3.5 are the recommended RAW settings set internally (launch channel, 2026-06-22; sample images regenerated at these values).

Independent of the other PRs — merge or skip on its own. (Turbo is unaffected: it always passes explicit --steps 8 --cfg 0.0.)