Validate a map object's pokemon field against the pokemon registry

R.maps.objects was f.opt(f.list(f.any)): a static wild encounter's
species (OverworldController.lua's d.pokemon, handed straight to
BattleState.newWild) went completely unchecked at load time, unlike an
encounter slot's species. A typo'd or removed id sat in a loaded mod
and only surfaced as a crash the moment a player reached that object.

Objects share one array across every kind -- NPCs, signs, warps and
static encounters all coexist with no field the loader could use to
tell them apart ahead of time -- so a strict f.rec covering the whole
shape would reject every kind this schema does not enumerate. Added
f.partial, an open counterpart to f.rec: it type-checks (and, through
collectRefs, cross-reference-checks) only the fields it is given and
leaves everything else on the value alone, the same extensibility
f.rec already grants at a record's top level but nowhere further in.
R.maps.objects now types just `pokemon` through it, so a bad species
id is a load-time "unresolved reference" error instead of a runtime
crash, while an NPC object's sprite/movement/range/... fields -- never
named in this schema -- still pass through untouched.

tests/engine/map_object_pokemon_ref_test.lua

@@ -0,0 +1,89 @@
+-- A map object's `pokemon` field (the static wild encounter kind --
+-- OverworldController.lua's `d.pokemon`, handed straight to
+-- BattleState.newWild with no existence check of its own) used to go
+-- completely unchecked: R.maps.objects was f.opt(f.list(f.any)), so a
+-- typo'd species sat in a loaded mod and only surfaced as a crash the
+-- moment a player stepped up to that object.  Every other kind sharing the
+-- objects array (NPCs, signs-as-objects, warps) has fields this schema
+-- still does not know about, which is what f.partial is for: it types only
+-- `pokemon` and leaves the rest of an object's shape alone.
+
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local T = require("tests.modkit")
+
+local function manifest(id)
+  return ([[{
+    "id": "%s", "name": "%s", "version": "1.0.0",
+    "entry": "main.lua", "api": 2
+  }]]):format(id, id)
+end
+
+-- ------- a bad species id is caught as a load error, not left to crash
+
+local BAD = {
+  ["mods/bad_static_encounter/manifest.json"] = manifest("bad_static_encounter"),
+  ["mods/bad_static_encounter/main.lua"] = [[
+    local mod = ...
+    mod.content.maps:patch("FIX_ROUTE", {
+      objects = {
+        { pokemon = "NOT_A_SPECIES", level = 30, text = "Gyaoo!" },
+      },
+    })
+  ]],
+}
+
+do
+  local run = T.sdk.loadMods({ "mods/bad_static_encounter" },
+    { fs = T.sdk.memfs(BAD) })
+  local dangling = {}
+  for _, message in ipairs(run.errors) do
+    if message:match("unresolved reference") then
+      dangling[#dangling + 1] = message
+    end
+  end
+  T.eq(#dangling, 1,
+    "a bad static-encounter species is reported once ("
+      .. table.concat(dangling, "; ") .. ")")
+  T.check(dangling[1] and dangling[1]:match("maps%.FIX_ROUTE%.objects")
+    and dangling[1]:match("pokemon"),
+    "the report names the map, the objects field and the pokemon registry: "
+      .. tostring(dangling[1]))
+  run.release()
+end
+
+-- ------- a real species resolves, and an NPC-shaped object beside it (no
+-- pokemon field at all, and fields this schema never named -- sprite,
+-- movement, range) is untouched
+
+local GOOD = {
+  ["mods/good_static_encounter/manifest.json"] = manifest("good_static_encounter"),
+  ["mods/good_static_encounter/main.lua"] = [[
+    local mod = ...
+    mod.content.maps:patch("FIX_ROUTE", {
+      objects = {
+        { index = 1, name = "FIXROUTE_TRAINER", sprite = "SPRITE_FIX_NPC",
+          movement = "STAY", range = "NONE", text = "TEXT_FIXROUTE_TRAINER",
+          x = 5, y = 9 },
+        { pokemon = "FIXMON_A", level = 30, text = "Gyaoo!" },
+      },
+    })
+  ]],
+}
+
+do
+  local run = T.sdk.loadMods({ "mods/good_static_encounter" },
+    { fs = T.sdk.memfs(GOOD) })
+  T.eq(#run.errors, 0,
+    "a real species and an untyped NPC object both load clean ("
+      .. tostring(run.errors[1]) .. ")")
+  local objects = run.data.maps.FIX_ROUTE.objects
+  T.eq(#objects, 2, "both objects landed on the map")
+  T.eq(objects[1].sprite, "SPRITE_FIX_NPC",
+    "the NPC object's untyped fields passed through unexamined")
+  T.eq(objects[2].pokemon, "FIXMON_A",
+    "the static encounter's species field passed through too")
+  run.release()
+end
+
+T.finish()