runtime, protocol: abortable() — per-call AbortSignal for host imports (A24)

[lannbot]

Closes #241. Implements contract amendment A24 — the advisory abort channel left open by #242.

The gap

A23 (#242) made a guest's cancellation of an in-flight host import return
promptly by discarding the result — but discard is about delivery, not
execution. The host operation keeps running: a discarded socket connect
keeps connecting, a discarded timer keeps its callback armed. For the
motivating consumer (polymorph-iroh's relay dial timeout) that's one
leaked connect per timeout, in a redial loop.

What A24 adds (contracts/embedder-api.md, in this diff)

abortable(fn) — the third per-declaration mark alongside suspending()
and deferCancel() (same direct-call + stage-3 decorator spellings, same
loud refusals, brand polyengine.abortable/1): every call to a marked
import receives a fresh AbortSignal appended after its WIT-declared
parameters, and the runtime aborts it when — and only when — that call's
subtask is discarded by a guest cancellation.

dial: abortable((addr: string, signal: AbortSignal) =>
  fetch(url, { signal }))

Design points:

• Per-call, not per-declaration. A function-level cancel() hook (the
  shape floated on A guest cannot cancel an in-flight host import: the subtask's on_cancel is a no-op #241) cannot say WHICH in-flight call died when several
  overlap; a per-call signal is right by construction, and it is the
  vocabulary the platform already accepts everywhere (fetch, sockets,
  timers).
• Signature-unconditional, abort-discard-only. A marked function
  always receives a signal — stable arity — but the signal only ever fires
  on a guest-initiated discard. Inert where discard cannot happen:
  sync-typed imports (no subtask handle), deferCancel imports
  (cancellation never discards), eager resolves, natural completion.
  Instance teardown deliberately does not abort (future amendment
  material).
• Deferred one microtask. The discard runs synchronously inside
  canon_subtask_cancel, i.e. inside a live guest activation; host abort
  listeners must not execute there (the issue-Guest callback invoked with null context slot (wit-bindgen async_support.rs:578) — polymorph-tls webcrypto-composed target #24 attribution class, plus
  re-entrancy). Promise.resolve().then, not queueMicrotask — the
  bare-engine-shell precedent from the jspi sentinels. Ordering pinned by
  test: the guest observes CANCELLED_BEFORE_RETURNED first, the host
  observes the abort a tick later, and an abort-provoked AbortError
  rejection lands on A23's resolved-subtask guards — discarded like any
  other late settlement.

The fix the review process earned

The conventions facade (#wrapImportFn) rebuilt host argument lists from
WIT arity, silently dropping anything past it — A24 would have been dead
through instantiate(), the only public entry point, with the signal
parameter forever undefined. The implementing agent caught it; trailing
runtime-minted extras are now forwarded verbatim (the loop is empty for
every unmarked import), and the embedder-layer abort test isolates exactly
that hunk: with the forwarding removed it fails loudly (the import body
traps on signal.addEventListener) while the raw-exec-layer test still
passes.

Tests

Protocol brand suite (independence: all three marks on one function);
host_import_cancel_test.ts +6 (signal arity and position, unmarked-import
arity control, the microtask deferral pinned from both sides —
aborted === false synchronously after the cancel returns, true one
tick later — AbortError inertness, deferCancel composition never aborts,
natural completion never aborts); fixture sleep-abort/cancel-abort/
run-abortable probes timed end-to-end at both layers, with the host
clearing its timer on abort. Three negative controls (abort removed;
deferral made synchronous; facade forwarding removed) each fail exactly
the tests that own them, verified with md5-checked restores.

Versioning and labels

Protocol changes ride the pending unpublished 0.2.3 (byte-identity guard
vacuous — no further bump). Goldens byte-identical. No breaking/* label:
additive protocol surface; the runtime hands marked imports an argument
they asked for and changes nothing for unmarked ones (pinned by the arity
control test).

Gates

just gates — exit 0: runtime 676/0, protocol 73/0, conformance 1257/0
(no expectation changes), sched-seeds both seeds, sm/node/bun pinned
shells, chromium + firefox all realms, smoke-tls, smoke-c0, npm/publish
checks, version-guard (reports protocol 0.2.3 as a pending bump).