Close #1035 Fix :> / :< generators producing values that violate the schema

[vssekorin]

Fixes #1035.

Problem

[:> x] turned the exclusive bound into an inclusive one with (inc x) and handed it to gen/double* as :min. inc does not move a double once |x| >= 2^53, so the generator was asked for values >= x and produced x itself:

(= Double/MAX_VALUE
   (shrink [:> Double/MAX_VALUE])
   (shrink [:>= Double/MAX_VALUE]))
;=> true

(> Double/MAX_VALUE Double/MAX_VALUE)
;=> false

The reported Double/MAX_VALUE case is just the extreme end of it — the bug starts at 2^53:

(->> (mg/sample [:> 1e16] {:seed 1 :size 100}) (remove (m/validator [:> 1e16])) count)
;=> 2
(->> (mg/sample [:> 1e300] {:seed 1 :size 100}) (remove (m/validator [:> 1e300])) count)
;=> 2

:< had the mirror problem via (dec x), and additionally threw on [:< (- Double/MAX_VALUE)] (Couldn't satisfy such-that predicate after 10 tries).

Fix

src/malli/generator.cljc

• -next-up — the next representable double above x. Math/nextUp on the JVM; in ClojureScript, where no such function exists, one arithmetic ulp step (overshooting a representable value or two is harmless for generation).
• gen-double-above — generator of doubles strictly greater than x, covering the three edges:
  • nothing is greater than ##NaN or ##Inf → unreachable generator (-never-gen);
  • everything finite is greater than ##-Inf → the plain double generator;
  • above Double/MAX_VALUE only ##Inf is left → (gen/return ##Inf).
• :> now delegates to gen-double-above; :< is :> negated, which removes the duplicated mirror logic and fixes [:< (- Double/MAX_VALUE)] along the way.

(mg/generate [:> Double/MAX_VALUE] {:seed 0})    ;=> ##Inf
(mg/generate [:> 1e16] {:seed 0})                ;=> 1.3510798882111488E16
(mg/generate [:< (- Double/MAX_VALUE)] {:seed 0}) ;=> ##-Inf
(mg/generate [:> 5] {:seed 0})                   ;=> 6.0   (unchanged)