Follow-ups to #4 normalizer PoC v2:
- Profile.float_decimals (default 6) flows into Rules.RoundFloatsInNode.
- mask_volatile_settings switches from name-only HashSet to a JSONPath-lite
allowlist ($.a.b.c) so same-named fields in unrelated subtrees stay intact.
- default.yaml migrated; 6 new tests including a regression trap for the
unrelated-subtree case. 16/16 normalizer tests, 77/77 solution tests.
Refs #2
