Host imports, scalar slice: a component can call Clojure through cljwit.host

  (host/instantiate art {:imports {"local:imp/host@0.1.0#twice" (fn [v] (* 2 v))}})
  ((i "run") 20)   ;=> 41

An upcall stub per import, built from the reflected import type the same way
export marshallers are, and bound to a Clojure reify. Missing imports stay
wasmtime's to report -- its message names the interface and the function, which
0017's first draft denied and one command disproved. What the host checks is
the other direction: a key nobody needs is a typo, and silently ignoring it is
the nil-on-typo failure 0014 rejected.

An exception in an import surfaces to the caller with its own message and marks
the instance dead. That is not politeness -- measured, such an error poisons the
whole store, so without the flag every later call reports "cannot enter
component instance" with the cause gone. Fault injection: removing the marking
turns the suite red, so the assertion is not decorative.

Scalars only, deliberately. Anything wasmtime would have to free -- a string, a
list, a boxed option -- needs lower-fn to allocate with malloc rather than from
an Arena (0017 C), and handing wasmtime an Arena pointer to free() is heap
corruption rather than a wrong answer. Refused at instantiate with the kind
named, the same shape the export direction uses.

97 assertions.

Claude-Session: https://claude.ai/code/session_01XF5Hfq4Ca2N2XYEzWQQuHt

Author: chaploud

dev/resources/imp.wit

@@ -1,7 +1,3 @@
-// A component that *imports* a function, so the guest can call back into the
-// host. `0014` names host imports as its most-likely-to-fire falsifier: WASI
-// needs them, and they break both its in-call flag and its reused argument
-// buffer, because a call can then nest.
 package local:imp@0.1.0;
 interface host { twice: func(v: u32) -> u32; }
 world w {