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-fixed.md

@@ -0,0 +1,26 @@
+---
+id: fn-fixed
+name: fixed
+category: builtins
+kind: builtin
+tokens: fixed
+sig: fixed(n) -> fixed
+tip: Lift an integer into a Q16.16 fixed-point value.
+order: 50
+---
+
+Converts an integer into a <code>fixed</code> value (Ludic's Q16.16 fixed-point type), so it can take part in fractional arithmetic. Ludic has no floating point; <code>fixed</code> is how you carry sub-pixel precision for smooth movement and physics-like accumulation. Use <code>fixed</code> when you need to combine an <code>int</code> with fixed-point values or divide to get a fraction — for example <code>fixed(1) / fixed(4)</code> is <code>0.25</code>. Convert back to a whole number for drawing with <code>flr</code>.
+
+Parameters:
+- `n` — the integer to lift into fixed-point
+
+```ludic
+program SmoothAccumulate {
+  handler ComputeStep phase Start {
+    let full_speed = fixed(3)
+    let half_speed = full_speed / fixed(2)
+    let two_steps = half_speed + half_speed
+    print(floor(two_steps))
+  }
+}
+```