refactor(lang): rename the str type (and stringify builtin) to string

Expand the abbreviated string type and its conversion builtin to the full
word everywhere:
  str            ->  string        (the immutable-string type)
  str(x) -> str  ->  string(x) -> string   (the stringify builtin;
                                            what `{…}` interpolation calls)

Types are recognized by identifier, and llty maps both spellings to LLVM
`ptr`, so this is an atomic source rewrite: type annotations, the Ludic
type tags, the builtin name/dispatch, and the interpolation desugar, plus
the grammars, LSP, docs (type-str -> type-string, fn-str -> fn-string), and
inventory. int/bool stay (universally accepted, like Math).

Reseeded; C-free fixpoint holds; all suites green (45/24/29); site + check.py OK.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/ecs/fn-world_field_id.md

@@ -27,7 +27,7 @@ program ResolveField {
   handler Inspect phase Update {
     let position_property = world_prop_id("Position")
     let column_field = world_field_id(position_property, "column")
-    Screen.status(str(column_field))
+    Screen.status(string(column_field))
   }
 }
 ```

docs/language/ecs/fn-world_get.md

@@ -30,7 +30,7 @@ program ReadField {
     let column_field = world_field_id(position_property, "column")
     for (position) in query [Position, {Player}] {
       let value = world_get(self(), position_property, column_field)
-      Screen.status(str(value))
+      Screen.status(string(value))
     }
   }
 }

docs/language/ecs/fn-world_model_id.md

@@ -25,7 +25,7 @@ program ResolveModel {
 
   handler Inspect phase Update {
     let player_model = world_model_id("Player")
-    Screen.status(str(player_model))
+    Screen.status(string(player_model))
   }
 }
 ```

docs/language/ecs/fn-world_prop_id.md

@@ -25,7 +25,7 @@ program ResolveProperty {
 
   handler Inspect phase Update {
     let position_property = world_prop_id("Position")
-    Screen.status(str(position_property))
+    Screen.status(string(position_property))
   }
 }
 ```

docs/language/ecs/fn-world_register_prop.md

@@ -23,7 +23,7 @@ program ModComponent {
   handler Boot phase Start {
     spawn Hero { Position { column: 4, row: 4 } }
     let stamina_property = world_register_prop("Stamina", 1)
-    Screen.status(str(stamina_property))
+    Screen.status(string(stamina_property))
   }
 }
 ```

docs/language/ecs/fn-world_save.md

@@ -27,7 +27,7 @@ program SnapshotOut {
     if Input.key() == 's' {
       let buffer = bytes(4096)
       let written = world_save(buffer)
-      Screen.status(str(written))
+      Screen.status(string(written))
     }
   }
 }

docs/language/ecs/fn-world_spawn.md

@@ -23,7 +23,7 @@ program Reinforcements {
     if Input.key() == ' ' {
       let enemy_model = world_model_id("Enemy")
       let new_enemy = world_spawn(enemy_model)
-      Screen.status(str(new_enemy))
+      Screen.status(string(new_enemy))
     }
   }
 }