feat: {:awaitable, fn} host-parallel capabilities + yield propagation through call args

[ivarvong]

Summary

• New {:awaitable, fn} callable variant: hosts register external tools as awaitable capabilities; calling one from Python returns a coroutine that yields an {:asyncio_capability_call, ...} sentinel for the trampoline to dispatch
• asyncio.gather over awaitable caps fans out all calls in a round as concurrent BEAM Tasks (Task.async_stream), then resumes each iter with its result — ~20x speedup for a batch of 20 × 50 ms calls
• out.append(await slow_call(i)) and any f(await expr, ...) pattern now works correctly; yield signals thread through call args via new {:cont_call_pos_resume, ...} / {:cont_call_kw_resume, ...} / star-arg frames
• advance_iter_one uses fresh semantics for resolved capability sentinels, fixing await helper() where helper wraps a cap (InvalidStateError: capability iter not awaiting in nested-await chains)

Architecture

{:awaitable, fn} capability flow

# Python code — identical regardless of sequential vs parallel
async def main():
    results = await asyncio.gather(*[search(q) for q in queries])

# Sequential (builtin) — Pyex drives one at a time
%{"tools" => %{"search" => {:builtin, fn [q] -> ... end}}}

# Parallel (awaitable) — gather dispatches all as BEAM Tasks concurrently  
%{"tools" => %{"search" => {:awaitable, fn [q] -> ... end}}}

The Python code is identical. The host registration controls fan-out shape.

Yield propagation through call args

eval_var_attr_call, eval_call_expr (getattr), and eval_call_expr (general) now route through Calls.eval_call_with_yield/5 → eval_remaining_and_call/8, which evaluates args recursively with full yield support and mutation write-back (:mutate, :mutate_arg handled per call-site variant).

advance_iter_one fix for resolved caps

When advance_iter_one is called on a :gen_pending iter whose buffered value is a capability sentinel AND that cap is already {:gen_done, _}, it uses fresh semantics (runs the continuation) rather than re-surfacing the stale sentinel. Regular sleep yields remain on the buffered path so cooperative interleaving (ABABAB gather test) is unaffected.

Test plan

• mix format --check-formatted clean
• mix compile --warnings-as-errors clean
• mix test — 5356 tests, 0 failures, 2 skipped
• mix dialyzer — 43 errors / 43 skipped (baseline unchanged)
• 7 new conformance tests covering: sequential await, gather order, parallel speedup, out.append(await cap(i)), gather vs loop identity, nested helper coroutine, gather over helpers

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