[0.42-r2 L1] prune disconnected shadow roots from __submitRoots after morph

[SisyphusZheng]

Severity: low · Category: implementation bug

Evidence. __submitRoots only grows; shadow roots removed by morphs stay referenced (listener + memory retention).

Fix. In __scanSubmitRoots, first prune entries whose root is no longer connected (!root.isConnected for ShadowRoot; keep document).

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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).