feat(lang): strict numbers in float files; render3d on float

A numbers float file adapts decimal literals to a fixed operand or slot, and refuses to
promote a computed int to a float implicitly: there it is almost always float bits. Explicit
float(x) is always allowed.

render3d's numbers are float, converted by tools/migrate/floatbits.py - a whole-program
inference of which ints carried IEEE bits (union-find over flows, calls, returns, buffers,
nested buffers and lexical scopes) and a rewriter to operators, Math.* and float literals,
with float_bits / float_from_bits left only where bits really cross (runtime scratch
buffers, mixed buffers). Seed regenerated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

selfhost/backend/emit_expr.ludic

@@ -109,6 +109,7 @@ function to_fixed(v: Val) -> pointer {
 # narrows with trunc; everything else (same width, or ptr) passes through.
 function coerce_code(v: Val, target: pointer) -> pointer {
   if is_fp(target) { return to_fp(v, target, `a {target} slot`) }
+  if (target == "fixed") and not (fixed_lit_code(v) == "") { return fixed_lit_code(v) }
   if is_fp(v.ty) and not is_fp(target) and (llty(target) != "ptr") {
     perr(`a {v.ty} does not convert to {target} implicitly — write int(x) or fixed(x)`)
   }