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>

docs/language/builtins/fn-flr.md

@@ -1,29 +0,0 @@
----
-id: fn-flr
-name: flr
-category: builtins
-kind: builtin
-tokens: flr
-sig: flr(x) -> int
-tip: Floor a fixed-point value down to the nearest integer.
-order: 50
----
-
-Converts a <code>fixed</code> (Q16.16) value back to an <code>int</code> by discarding the fractional part, rounding toward negative infinity. It is the counterpart to <code>fx</code>: you accumulate motion in fixed-point for sub-pixel smoothness, then <code>flr</code> the result to get the whole-pixel column or row to draw at. Because it floors rather than rounds, <code>flr(fx(3) / fx(2))</code> is <code>1</code>, not <code>2</code>. Use it wherever a fixed value must become an integer coordinate, count, or index.
-
-Parameters:
-- `x` — the fixed-point value to floor
-
-```ludic
-program FixedToPixels {
-  const TILE_SIZE: int = 16
-
-  handler DrawWorld phase Render {
-    Screen.clear(Color.MidnightBlue)
-    let smooth_column = fx(5) + fx(1) / fx(2)
-    let pixel_x = flr(smooth_column) * TILE_SIZE
-    Screen.fill_rectangle(x: pixel_x, y: 32, width: TILE_SIZE, height: TILE_SIZE, color: Color.LimeGreen)
-    Screen.show()
-  }
-}
-```