Chez Phase 0a+0b: value model + host-neutral contract gate

0a (host/chez/values.ss): Jolt value model on Chez — nil sentinel distinct from
#f/'(), interned keywords, ns+meta symbols, exactness-aware = and consistent
hash. Chez numeric tower gives ratios/bignums free. 33/33 tests.

0b (test/chez/): extract test/spec/*.janet defspec tables into corpus.edn (2655
cases, valid as both EDN and Janet data), and a runner that drives ANY jolt
binary via the CLI boundary with per-case subprocess isolation. Pluggable target
(JOLT_BIN) so the same corpus gates every host. Baseline vs Janet build/jolt:
2641/2655, 14 known CLI divergences allowlisted; gate fails only on NEW ones.

test/chez/known-divergences.edn

@@ -0,0 +1,14 @@
+["max non-number throws"
+ "min-key keys nonnum"
+ "neg? keyword"
+ "< nil"
+ "> with nil"
+ "max non-number"
+ "neg? throws"
+ "vector index"
+ "map key as fn"
+ "map key default"
+ "set membership"
+ "set miss default"
+ "collection key"
+ "xml-seq walks"]