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Fix PyPvmlPynterEvaluator hanging forever on WASM instantiation failure (#334)

Emscripten's generated createWasm() (pynterwasm.js) only resolves via
the success callback passed to Module.instantiateWasm — it never
observes the promise our own instantiateWasm returns. So whenever
wasm(imports) rejects (unsupported WASM, a fetch/network failure, a
malformed binary, ...), the success callback never fires and the
whole pynterwasm(...)/init() call hangs forever with no error ever
surfaced, leaving evaluateChunk() awaiting it indefinitely instead of
reporting a clean error via sendError.

Race init() against a promise we control so a genuine instantiation
failure rejects it instead of hanging. Add PyPvmlPynterEvaluator.test.ts,
which also documents (in Jest, where WASM is stubbed out — see
src/tests/__mocks__/wasm.js) that each evaluateChunk() call is fully
self-contained: unlike PyPvmlEvaluator/PyCseEvaluator, this evaluator
does not persist a global environment across calls, so it only
supports single-shot "run this whole program" usage, not REPL-style
incremental development. Documented with a comment on the class too,
flagging it for review before deciding how to expose this evaluator
(e.g. in language-directory).


Claude-Session: https://claude.ai/code/session_01KrwJGug3rCXijRseRjys9V

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

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