Chez Phase 1 (increment 3h): host-interop method-call emit

(.method target arg*) now analyzes to a :host-call IR node instead of
punting at analyze. The Chez back end lowers it to a jolt-host-call
dispatch for the methods the RT shims (.write -> port display,
.isDirectory -> file-directory?, .listFiles -> directory-list); any
other method stays out of subset (clean emit-time reject, so it can't
read as a compiled-but-broken corpus divergence). The Janet back end
punts ALL :host-call to the interpreter, same shape as letfn: compiles
on Chez, interprets on Janet, zero change to the main language.

Closes the io tier's print-method defmethods and file-seq: prelude emit
reach 348 -> 354/355 (50-io 20/20). The one remaining gap is the regex
literal in parse-uuid (needs a regex engine on Chez; deferred).

emit-test 122/122; Chez subset 672/672, 0 divergences; full gate green.

src/jolt/backend.janet

@@ -777,6 +777,10 @@
       :map (emit-map ctx node)
       :set (emit-set ctx node)
       :quote ['quote (node :form)]
+      # host interop (.method target ...): the back end doesn't model interop —
+      # punt to the interpreter, exactly as the analyzer used to before producing
+      # a :host-call node (the Chez back end lowers it instead).
+      :host-call (error "jolt/uncompilable: host method call")
       (error (string "backend: unhandled op " (node :op))))))
 
 (defn emit-ir