embedder: propagate producer failures to pending reads

[lannbot]

Summary

Fixes #325.

• Surface the recorded site-named producer failure on pending stream reads, iterator/readable consumers, and unfinished direct-read sessions.
• Retire failed futures through the existing abandonment path, waking readers and releasing host activity while preserving the producer cause.
• Preserve completed payloads and direct sessions, and keep unrelated read failures primary.

Regression coverage includes failure before/after CABI binding, parked host operations, guest future abandonment, error identity, activity cleanup, and completed-result races.

Verification

• just gates < /dev/null passed, including browser and consumer smokes.
• Focused embedder suites: 71 passed, no skips.
• Independent review approved after error-precedence and race-test corrections.

Automerge will be armed with a merge commit after opening; required core checks gate the merge.