feat: build-suffix labels for [add] actions; chain RV Interior Expansion after Project RV B42

Label format change ('!missing' actions):
  - "add TrueMoozic (renamed from truemusic)" → "add TrueMoozic B42"
  - "add truemusic"                           → "add truemusic B41"
Build context lives in the suffix; the alias-rename hint was redundant
because the user already sees the original mod_id in the warning text.

RV Interior Expansion ordering (B42):
  PROJECTRVInterior42 → RVInteriorExpansion → RVInteriorExpansionPart2
  map folders: map_distanciado → map_aquatsar → rvupdate → rv2 → Muldraugh, KY
Authors didn't declare loadAfter in mod.info; new modpack rules file
data/modpack_rules/rv_expansion.txt establishes the chain. Triggers fire
when either expansion wsid (3618427553 / 3622163276) is in input.

api/app.py

@@ -342,11 +342,18 @@ _MODPACK_RULES_TRIGGERS: Dict[str, str] = {
     # HellDrinx FULL + LITE both bundle the same sorting_rules.txt.
     "3672556207": "helldrinx.txt",  # HellDrinx FULL
     "3662909244": "helldrinx.txt",  # HellDrinx LITE
+    # RVInteriorExpansion + Part2 chain after PROJECTRVInterior42.
+    # Expansion authors didn't declare loadAfter in their mod.info; without
+    # these rules sortof has no signal to chain the cluster.
+    "3618427553": "rv_expansion.txt",  # RVInteriorExpansion (rvupdate)
+    "3622163276": "rv_expansion.txt",  # RVInteriorExpansionPart2 (rv2)
 }
 # Human-readable trigger labels for the warning message.
 _MODPACK_RULES_LABELS: Dict[str, str] = {
     "3672556207": "HellDrinx FULL",
     "3662909244": "HellDrinx LITE",
+    "3618427553": "RV Interior Expansion",
+    "3622163276": "RV Interior Expansion Part 2",
 }