[0.42-r2 M4] forms inside late-hydrating (idle/visible) islands never get the enhancement listener

[SisyphusZheng]

Severity: medium · Category: implementation bug

Evidence. __scanSubmitRoots runs at ready and after morphs. client:idle/client:visible islands create their shadow roots AFTER the ready scan; an enhanced form inside such an island has no submit listener and always posts natively.

Fix. After each island module load resolves, schedule a rescan (setTimeout 0) so newly-upgraded shadow roots get the listener.

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Filed from the 0.42 round-2 review (docs/audit/2026-07-28-alpha5-round2-review.md). Target line: 0.42.0-alpha.6.

[SisyphusZheng]

Fixed and shipped in v0.42.0-alpha.6 (npm dist-tags latest/alpha at 0.42.0-alpha.6, tag ccc9407). The fix and its regression test landed via PR #594 (fixture suite 49 tests × 3 engines = 147 green, including the new round-2 specs; unit + parity suites green).