refactor: data/modpack_rules → data/rules; auto-apply on resort path

The directory was misnamed — these are wsid-triggered sorting-rule
overlays, not modpack-specific. HellDrinx happens to be a modpack but
RV Interior Expansion isn't; both use the same generic mechanism.

Renames:
  data/modpack_rules/        → data/rules/
  _MODPACK_RULES_*           → _RULES_*
  _modpack_rules_for         → _auto_rules_for
  _parse_rules_with_modpacks → _parse_rules_combined
  _emit_modpack_rules_warnings → _emit_rules_applied_warnings
  warning tag "modpack-rules-applied" → "rules-applied"

Bug fix: /api/resort was passing {} rules to sort_mods, silently
dropping every auto-injected rule. The frontend runs an automatic
resort after each /api/sort (driven by localStorage branchSelections),
so the user always saw the resort output — where rv_expansion.txt's
loadAfter chain wasn't taking effect. Now resort goes through
_parse_rules_combined and emits rules-applied warnings the same way
/api/sort does.

data/rules/rv_expansion.txt

@@ -0,0 +1,19 @@
+; RV Interior Expansion (B42) — sorting rules
+; Triggers:
+;   - 3618427553 (RVInteriorExpansion, map folder rvupdate)
+;   - 3622163276 (RVInteriorExpansionPart2, map folder rv2)
+; Auto-injected by app.py:_auto_rules_for() when either trigger wsid is
+; in the user's input. User-supplied rules are appended afterward and
+; override these on conflicting keys.
+;
+; Authored ordering: PROJECTRVInterior42 → RVInteriorExpansion → RVInteriorExpansionPart2.
+; The expansion mods don't declare these loadAfter relationships in their own
+; mod.info, so without these rules sortof has nothing to base the cluster
+; ordering on (PROJECTRVInterior42 is in PREORDER slot 7 already; expansions
+; just need to chain off of it).
+
+[RVInteriorExpansion]
+loadAfter=PROJECTRVInterior42
+
+[RVInteriorExpansionPart2]
+loadAfter=RVInteriorExpansion