refactor(lang): rename builtins flr->floor and fx->fixed

De-abbreviate the two bare fixed-point conversion builtins:
  flr(f) -> int     ->  floor(f) -> int    (fixed -> int, flooring)
  fx(i)  -> fixed   ->  fixed(i) -> fixed  (int -> fixed; mirrors how the
                                            stringify builtin is `string`)

Updates the compiler dispatch, all call sites, the grammars/LSP/JetBrains
tokens, and the docs (fn-flr -> fn-floor, fn-fx -> fn-fixed). 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>

tools/docgen/inventory.json

@@ -35,8 +35,8 @@
     "fn-max",
     "fn-abs",
     "fn-clamp",
-    "fn-fx",
-    "fn-flr",
+    "fn-fixed",
+    "fn-floor",
     "fn-bytes",
     "fn-words",
     "fn-ui_build",