runtime: separate call completion from store driver outcomes

[lannbot]

Summary

Fixes #350. Fixes #357.

Cycle 1 of the requested two-cycle scheduler refactor: calls own their outcomes while the existing store drivers continue owning execution progress.

• Separate canonical result capture from delivery at the owning activation's return or genuine JSPI suspension. Unrelated host work no longer withholds a result; synchronous post-return remains inside its invocation's failure boundary.
• Route autonomous scheduler faults to their originating host call, including nested FACT roots, rather than whichever export or host activity happens to drive the store.
• Preserve already-published results, synchronous return shape, eager Future conventions, raw non-poisoning host reports, and background progress after a routed fault.
• Make poisoning/cleanup notification preserve the first cause (including undefined) and attempt every affected call despite cleanup failures.
• Add real plain/JSPI two-instance and nested-FACT regressions, plus ready-only handoff, synchronous shape, pre-entry report, and cleanup/poison tests.

Cycle 2 will replace overlapping driver/arrival/fallback-pump machinery with event-triggered draining. This PR intentionally keeps that machinery while establishing its behavioral contract.

Verification

• Original boundary: plain mode — a completed async export stays pending while an unrelated host call is outstanding #350/boundary: a sibling instance's trap, ticked by this export's driver, rejects this export although its task already returned #357 scripts fail on the baseline and produce the expected results after this change.
• Independent review found and drove corrections for nested origin delivery, post-publication fault contamination, synchronous shape, and ready-only handoff; final review cleared the diff.
• Final just gates < /dev/null passed: runtime 1006 passed / 3 existing ignored; WASI 305 passed / 1 optional fixture ignored; ct-runner 24 passed / 1 optional fixture ignored; seeded runtime 1003 passed / 6 existing ignored per seed; seeded conformance 95 passed / 1 profile exclusion.
• Formatting, lint, native tests, packaging, conformance, required shell/browser lanes, Chromium/Firefox OPFS, and available consumer smokes passed. Existing arm64 JSC unavailability and conformance expectations remain. TLS/C0 smokes include their existing not-applicable and translation-only cases.
• Final targeted independent review: 5 passed; diff whitespace checks passed.

No protocol marks, manifests, goldens, or conformance expectations changed. Automerge will be armed and watched through delivery.