feat(perf): ship ModernFix dynamic_resources, add Flerovium and Iris Flywheel Compat

Bench (proxy 9900X/7800XT, see analysis/client-perf/RESULTS-proxy.md):
- ModernFix mixin.perf.dynamic_resources=true: startup -7.6 s, S1 p99
  9.11 -> 8.48 ms, S2 1% low 236 -> 264 fps (B6). Enforced packwiz file
  because ModernFix reads it before Default Options. Missing-texture check
  waived by owner; issues to be reported from play.
- Flerovium 1.1.2 (client): mob-pen S3 p99 ~0.9 ms better (B12b).
- Iris Flywheel Compat 2.4.0 (client): with a shaderpack, Create floor
  p99 13.3 -> 4.0 ms (Flywheel instancing kept under Iris) (B12b).
Deck tier already ships DH's LOW quality preset, so no DH change.

.packwizignore

@@ -70,3 +70,6 @@ config/paxi/datapacks/*
 !config/alexsmobs-common.toml
 !config/fowlplay.json5
 !config/crittersandcompanions-common.json5
+# ModernFix mixin overrides (build.py SHIP; enforced - ModernFix reads it before
+# Default Options runs). dynamic_resources=true per bench B6.
+!config/modernfix-mixins.properties