feat(lang): float and double types with ordinary operators

`float` (32-bit) and `double` (64-bit) with + - * / %, comparisons and unary minus.
Decimal literals take their type from context and stay `fixed` elsewhere; int and long
promote implicitly (LUDIC_WARN_FLOAT_PROMOTE=1 lists every promotion). float(), double(),
int(), long() and fixed() convert; floats(n)/doubles(n) buffers; float fields, globals,
constants and parameters; Math.* computes in float for float arguments; string/print
write the shortest round-tripping decimal; float_bits/float_from_bits expose the bits.
@deterministic code may not use floats. The f_* runtime helpers stay as they are.

Editors know the new type words; the JetBrains plugin is 1.5.0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/language/builtins/fn-float.md

@@ -0,0 +1,25 @@
+---
+id: fn-float
+name: float
+category: builtins
+kind: builtin
+tokens: float double
+sig: float(x) -> float / double(x) -> double
+tip: Convert any number to a float (or double).
+order: 51
+---
+
+Converts an <code>int</code>, <code>long</code>, <code>fixed</code>, <code>float</code> or <code>double</code> to a <code>float</code> (or, with <code>double(x)</code>, to a <code>double</code>). It is the one conversion that crosses from fixed-point to floating point: `float(fixed(1)) / 4.0` is <code>0.25</code>. A decimal literal converts exactly, so `float(0.1)` is the nearest float to 0.1, not the nearest Q16.16 value.
+
+Parameters:
+- `x` — the number to convert
+
+```ludic
+program Convert {
+  entry {
+    let half = fixed(1) / 2
+    print(float(half) * 3.0)           # 1.5
+    print(double(7) / 2)               # 3.5
+  }
+}
+```