Pass exec/2 commands to the javascript host in the wasm version

[jeswr]

What

Wraps exec/2 in the same :- if(current_prolog_flag(emscripten, true)) conditional already used for userInput/2, so that in wasm builds the sub-reasoner invocations of the graph scoped builtins are passed to the JavaScript host via await/2 instead of calling shell/2 (which does not exist under Emscripten). Native builds keep the existing shell/2 definition verbatim.

This is the small patch proposed in eyereasoner/eye-js#873 ("Left for @josd" / upstream ask): all eight graph-literal-scoped builtins — log:collectAllIn, log:forAllIn, log:ifThenElseIn, log:conclusion, log:satisfiable, e:call, e:fail, e:findall — funnel through exec/2, so this one conditional makes them all bridgeable for any wasm embedder, and makes the sub-reasoner-spawn contract explicit rather than something each embedder has to shim in at image-build time.

Host protocol

Per your preference for a structured term over a raw shell string, the emscripten branch splits the command line and yields

await(exec(Args, File), Response)

where Args is the argv of the command as a list of atoms (e.g. [eye, '--nope', '--quiet', Tmp1, '--query', Tmp2]) and File is the target of its stdout redirection. The host is expected to run the command — for the graph scoped builtins, the eye sub-reasoner on a fresh wasm instance, which is the isolation analogue of the process spawn — write its standard output to File in the requesting instance's filesystem, and answer "ok". Any other response, and any command without a stdout redirection (e.g. arbitrary e:exec shell commands, which a wasm host cannot honour), raises exec_error(A) just like a non-zero exit does natively; the scoped builtins already wrap exec/2 in catch/3 and treat that as failure.

Validation

No native swipl was available here, so validation ran under swipl-wasm (where the emscripten branch is live and both conditional branches must parse):

• consulting the patched eye.pl in swipl-wasm produces no errors or warnings;
• the exact repro from log:collectAllIn error eye-js#873 (graph-literal-scoped log:collectAllIn), run through qsave_programd images of this patched source with a JS host answering the exec(Args, File) yields by booting fresh sub-instances, returns (:x) a :Result. — identical to native EYE, with the expected two sub-reasoner invocations per scope evaluation.

The native branch is byte-identical to the current definition, so non-wasm behaviour is unchanged; a native CI run is still the authoritative check there.

Reference host

eye-js is the reference consumer: eyereasoner/eye-js#1956 currently injects an equivalent (string-based) redefinition of exec/2 at pvm-generation time; once a release contains this patch, eye-js can drop that build-time shim and consume the structured yield directly (a small follow-up on their side). VERSION/RELEASE/eye.zip are deliberately untouched — left to your release flow.

Prepared agent-assisted for @jeswr.

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